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@article{H.Takahashi_PRBL_2024,

title = {Pressure evolution of the normal- and superconducting-state properties of the line-nodal material CaSb2 revealed by 123Sb nuclear quadrupole resonance},

author = {H Takahashi and S Kitagawa and K Ishida and A Ikeda and S R Saha and S Yonezawa and J Paglione and Y Maeno},

url = {https://arxiv.org/abs/2402.12783},

doi = {10.1103/PhysRevB.109.L100501},

year  = {2024},

date = {2024-03-04},

urldate = {2024-03-04},

journal = {Physical Review B},

volume = {109},

pages = {L100501},

abstract = {CaSb 2 is the Dirac line-nodal material that exhibits a superconducting (SC) transition at 1.7 K. In spite of its conventional SC state at ambient pressure, the transition temperature T c shows a peak structure against hydrostatic pressure. We performed ac magnetic susceptibility and 123 Sb nuclear quadrupole resonance (NQR) measurements on single-crystalline CaSb 2 under pressures up to 2.08 GPa. T c monotonically increased in this pressure region, which is consistent with a previous study. We observed continuous broadening of the NQR spectrum against pressure, which is a sign of unique compression behavior of the lattice. In the normal state, the nuclear spin-lattice relaxation rate 1 / T 1 is proportional to temperature in all pressure values, typical of a metal. However, 1 / T 1 T in the normal state is independent of pressure, indicating that the density of states at the Fermi energy N ( E F ) , which is one of the parameters governing T c , is insensitive to pressure. From these results, we conclude that N ( E F ) does not govern the origin of the enhancement in T c . This is unusual for a weak electron-phonon coupling superconductor. In the SC state, we revealed that the SC gap becomes larger and more isotropic under pressure.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{S.Kitagawa_PRB_2024,

title = {Breakdown of Kubo relation in Pt-Cu nanoparticles},

author = {S Kitagawa and Y Kinoshita and K Ishida and K Kusada and H Kitagawa},

doi = {10.1103/PhysRevB.109.L041408},

year  = {2024},

date = {2024-01-26},

urldate = {2024-01-26},

journal = {Physical Review B},

volume = {109},

pages = {L041408},

abstract = {Nanoparticles were predicted to exhibit unique physical properties due to quantum size effects, but their identification remains difficult.

According to Kubo's theory, the gap size is inversely correlated with both the density of states at the Fermi energy and the number of atoms in the particle.

Previously, we confirmed that the particle size and magnetic field dependence of NMR anomaly temperature is consistent with the estimated ``Kubo'' gap.

Here, we investigated the density-of-states dependence in the Pt$_{1-x}$Cu$_{x}$ nanoparticles.

While an enhancement of nuclear spin-lattice relaxation rate $1/T_1$ at low temperatures was clearly observed for the Pt-rich nanoparticles, such behavior was abruptly suppressed in the Cu-rich nanoparticles.

Furthermore, the NMR anomaly temperature is nearly unchanged with varying the density of states.

Our findings indicate that the quantum size effect contains more profound physics than just the ones predicted by Kubo.

},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Y.Tokunaga_PRL_2023,

title = {Longitudinal Spin Fluctuations Driving Field-Reinforced Superconductivity in  UTe2},

author = {Y Tokunaga and H Sakai and S Kambe and P Opletal and Y Tokiwa and Y Haga and S Kitagawa and K Ishida and D Aoki and G Knebel and G Lapertot and S Krämer and M. Horvatić},

url = {https://arxiv.org/abs/2307.10724},

doi = {10.1103/PhysRevLett.131.226503},

year  = {2023},

date = {2023-11-29},

urldate = {2023-11-29},

journal = {Physical Review Letters},

volume = {131},

pages = {226503},

abstract = {Our measurements of 125 Te NMR relaxations reveal an enhancement of electronic spin fluctuations above μ0H∗ ∼ 15 T, leading to their divergence in the vicinity of the metamagnetic transition at μ0Hm ≈ 35 T, below which field-reinforced superconductivity appears when a magnetic field(H) is applied along the crystallographic b axis. 

The NMR data evidence that these fluctuations are dominantly longitudinal, providing a key to understanding the peculiar superconducting phase diagram in H ∥ b, where such fluctuations enhance the pairing interactions.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Large perpendicular magnetic anisotropy induced by an inter-site charge transfer in strained EuVO2H films},

author = {M Namba and H Takatsu and R Mikita and Y Sijia and K Murayama and H-B Li and R Terada and C Tassel and H Ubukata and M Ochi and R S Puche and E P Latasa and N Ishimatsu and D Shiga and H Kumigashira and K Kinjo and S Kitagawa and K Ishida and T Terashima and K Fujita and T Mashiko and K Yanagisawa and K Kimoto and H Kageyama},

doi = {10.1021/jacs.3c04521},

year  = {2023},

date = {2023-09-28},

urldate = {2023-09-28},

journal = {Journal of the American Chemical Society　},

volume = {145},

pages = {21807},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Superconducting spin reorientation in spin-triplet multiple superconducting phases of UTe2},

author = {K Kinjo and H Fujibayashi and H Matsumura and F Hori and S Kitagawa and K Ishida and Y Tokunaga and H Sakai and S Kambe and A Nakamura and Y Shimizu and Y Homma and D Li and F Honda and D Aoki},

url = {https://arxiv.org/abs/2307.15784},

doi = {10.1126/sciadv.adg2736},

year  = {2023},

date = {2023-07-28},

urldate = {2023-07-28},

journal = {Science Advances},

volume = {9},

pages = {adg2736},

abstract = {Superconducting (SC) state has spin and orbital degrees of freedom, and spin-triplet superconductivity shows multiple SC phases because of the presence of these degrees of freedom. However, the observation of spin-direction rotation occurring inside the SC state (SC spin rotation) has hardly been reported. Uranium ditelluride, a recently found topological superconductor, exhibits various SC phases under pressure: SC state at ambient pressure (SC1), high-temperature SC state above 0.5 gigapascal (SC2), and low-temperature SC state above 0.5 gigapascal (SC3). We performed nuclear magnetic resonance (NMR) and ac susceptibility measurements on a single-crystal uranium ditelluride. The b axis spin susceptibility remains unchanged in SC2, unlike in SC1, and decreases below the SC2-SC3 transition with spin modulation. These unique properties in SC3 arise from the coexistence of two SC order parameters. Our NMR results confirm spin-triplet superconductivity with SC spin parallel to b axis in SC2 and unveil the remaining of spin degrees of freedom in SC uranium ditelluride.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Gapless fermionic excitation in the antiferromagnetic state of ytterbium zigzag chain},

author = {F Hori and K Kinjo and S Kitagawa and K Ishida and S Mizutani and R Yamamoto and Y Ohmagari and T Onimaru },

url = {https://arxiv.org/abs/2201.07563},

doi = {10.1038/s43246-023-00381-4},

year  = {2023},

date = {2023-07-22},

urldate = {2023-07-22},

journal = {Communications Materials},

volume = {4},

pages = {55},

abstract = {The emergence of charge-neutral fermionic excitations in magnetic systems is one of the unresolved issues in recent condensed matter physics. This type of excitations has been observed in various systems, such as low-dimensional quantum spin liquids, Kondo insulators, and antiferromagnetic insulators. Here, we report the presence of a pronounced gapless spin excitation in the low-temperature antiferromagnetic state of YbCuS2 semiconductor, where trivalent ytterbium atoms form a zigzag chain structure. We confirm the presence of this gapless excitations by a combination of experimental probes, namely 63/65Cu-nuclear magnetic resonance and nuclear quadrupole resonance, as well as specific heat measurements, revealing a linear low-temperature behavior of both the nuclear spin-lattice relaxation rate 1/T1 and the specific heat. This system provides a platform to investigate the origin of gapless excitations in spin chains and the relationship between emergent fermionic excitations and frustration.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{H.Matsumura_JPSJ_2023,

title = {Large Reduction in the a-axis Knight Shift on UTe2 with Tc = 2.1 K},

author = {H Matsumura and H Fujibayashi and K Kinjo and S Kitagawa and K Ishida and Y Tokunaga and H Sakai and S Kambe and A Nakamura and Y Shimizu and Y Homma and D Li and F Honda and D Aoki },

url = {https://arxiv.org/abs/2305.01200},

doi = {10.7566/JPSJ.92.063701},

year  = {2023},

date = {2023-05-16},

urldate = {2023-05-16},

journal = {Journal of the Physical Society of Japan},

volume = {92},

pages = {063701},

abstract = {Spin susceptibility in the superconducting (SC) state was measured in the higher-quality sample of uranium-based superconductor UTe2 by using Knight-shift measurements for a magnetic field H along all three crystalline axes. In the higher-quality sample, the SC transition temperature Tc is about 2.1 K, and the residual electronic term in the specific heat is almost zero. The NMR linewidth becomes narrower and is almost half of that in the previous sample with Tc ∼ 1.6 K when H || a and c. Although the Knight-shift behavior was not so different from the previous results for H || b, and c, a large reduction in Knight shift along the a axis was observed, in contrast with the previous a-axis Knight shift result. We discuss the origin of the difference between the previous and present results, and the possible SC state derived from the present results.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{H.Fujibayashi_JPSJ_2023,

title = {Low-Temperature Magnetic Fluctuations Investigated by 125Te-NMR on the Uranium-Based Superconductor UTe2},

author = {H Fujibayashi and K Kinjo and G Nakamine and S Kitagawa and K Ishida and Y Tokunaga and H Sakai and S Kambe and A Nakamura and Y Shimizu and Y Homma and D Li and F Honda and D Aoki},

doi = {10.7566/JPSJ.92.053702},

year  = {2023},

date = {2023-04-26},

urldate = {2023-04-26},

journal = {Journal of the Physical Society of Japan},

volume = {92},

pages = {053702},

abstract = {To investigate the static and dynamic magnetic properties on the uranium-based superconductor UTe2, we measured the NMR Knight shift K and the nuclear spin–lattice relaxation rate 1/T1 in H || a by 125Te-NMR on a 125Te-enriched single-crystal sample. 1/T1T in H || a is much smaller than 1/T1T in H || b and c, and magnetic fluctuations along each axis are derived from the 1/T1T measured in H parallel to all three crystalline axes. The magnetic fluctuations are almost identical at two Te sites and isotropic at high temperatures, but become anisotropic below 40 K, where heavy-fermion state is formed. The character of magnetic fluctuations in UTe2 is discussed with the comparison to its static susceptibility and the results on other U-based superconductors. It is considered that the magnetic fluctuations probed with the NMR measurements are determined by the magnetic properties inside the two-leg ladder formed by U atoms, which are dominated by the qa = 0 ferromagnetic fluctuations.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{S.Ogata_PRL_2023,

title = {Parity transition of spin-singlet superconductivity using sub-lattice degrees of freedom},

author = {S Ogata and S Kitagawa and K Kinjo and K Ishida and M Brando and E Hassinger and C Geibel and S Khim},

url = {https://arxiv.org/abs/2304.10032},

doi = {10.1103/PhysRevLett.130.166001},

year  = {2023},

date = {2023-04-19},

urldate = {2023-04-19},

journal = {Physical Review Letters},

volume = {130},

pages = {166001 },

abstract = {Recently, a superconducting (SC) transition from low-field (LF) to high-field (HF) SC states was reported in CeRh2As2, indicating the existence of multiple SC states. It has been theoretically noted that the existence of two Ce sites in the unit cell, the so-called sub-lattice degrees of freedom owing to the local inversion symmetry breaking at the Ce sites, can lead to the appearance of multiple SC phases even under an interaction inducing spin-singlet superconductivity. CeRh2As2 is considered as the first example of multiple SC phases owing to this sub-lattice degree of freedom. However, microscopic information about the SC states has not yet been reported. In this study, we measured the SC spin susceptibility at two crystallographically inequivalent As sites using nuclear magnetic resonance for various magnetic fields. Our experimental results strongly indicate a spin-singlet state in both SC phases. In addition, the antiferromagnetic phase, which appears within the SC phase, only coexists with the LF SC phase; there is no sign of magnetic ordering in the HF SC phase. The present work reveals unique SC properties originating from the locally noncentrosymmetric characteristics. },

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{K.Kinjo_PRB_2023,

title = {Change of superconducting character in UTe2 induced by magnetic field},

author = {K Kinjo and H Fujibayashi and S Kitagawa and K Ishida and Y Tokunaga and H Sakai and S Kambe and A Nakamura and Y Shimizu and Y Homma and D. X Li and F Honda and D Aoki and K Hiraki and M Kimata and T Sasaki},

url = {https://arxiv.org/abs/2206.02444},

doi = {10.1103/PhysRevB.107.L060502},

year  = {2023},

date = {2023-02-10},

urldate = {2023-02-10},

journal = {Physical Review B},

volume = {107},

issue = {06},

pages = {L060502},

abstract = {UTe2 is a recently discovered spin-triplet superconductor. One of the characteristic features of UTe2 is a magnetic field (H)-boosted superconductivity >16 T when H is applied exactly parallel to the b axis. To date, this superconducting (SC) state has not been thoroughly investigated, and the SC properties as well as the spin state of this high-H SC (HHSC) phase are not well understood. In this letter, we performed AC magnetic susceptibility and nuclear magnetic resonance measurements and found that, up to 24.8 T, the HHSC state has bulk nature and is quite sensitive to the H angle and that its SC character is different from that in the low-H SC (LHSC) state. The dominant spin component of the spin-triplet pair is along the a axis in the LHSC state but is changed in the HHSC state along the b axis. Our results indicate that H -induced multiple SC states originate from the remaining spin degrees of freedom.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{CaSb2ARPES,

title = {Fermiology of a topological line-nodal compound  CaSb2  and its implication to superconductivity: Angle-resolved photoemission study},

author = {C-Wen Chuang and S Souma and A Moriya and K Nakayama and A Ikeda and M Kawaguchi and K Obata and S Ranjan Saha and H Takahashi and S Kitagawa and K Ishida and K Tanaka and M Kitamura and K Horiba and H Kumigashira and T Takahashi and S Yonezawa and J Paglione and Y Maeno and T Sato},

url = {https://arxiv.org/abs/2211.15083},

doi = {10.1103/PhysRevMaterials.6.104203},

year  = {2022},

date = {2022-10-24},

urldate = {2022-10-24},

journal = {Physical Review Materials},

volume = {6},

issue = {10},

pages = {104203},

abstract = {We performed angle-resolved photoemission spectroscopy with microfocused beam on a topological line-nodal compound CaSb2 which undergoes a superconducting transition at the onset Tc ∼ 1.8 K, to clarify the Fermi-surface topology relevant to the occurrence of superconductivity. We found that a three-dimensional hole pocket at the Γ point is commonly seen for two types of single-crystalline samples fabricated by different growth conditions. On the other hand, the carrier-doping level estimated from the position of the chemical potential was found to be sensitive to the sample fabrication condition. The cylindrical electron pocket at the Y(C) point predicted by the calculations is absent in one of the two samples, despite the fact that both samples commonly show superconductivity with similar Tc's. This suggests a key role of the three-dimensional hole pocket to the occurrence of superconductivity, and further points to an intriguing possibility to control the topological nature of superconductivity by carrier tuning in CaSb2.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{M.Manago_JPSJ_2022,

title = {Site Split of Antiferromagnetic α-Mn Revealed by 55Mn Nuclear Magnetic Resonance},

author = {M Manago and G Motoyama and S Nishigori and K Fujiwara and K Kinjo and S Kitagawa and K Ishida and K Akiba and S Araki and T C Kobayashi and Hisatomo Harima},

url = {https://arxiv.org/abs/2210.02754},

doi = {10.7566/JPSJ.91.113701},

year  = {2022},

date = {2022-10-06},

urldate = {2022-10-06},

journal = {Journal of the Physical Society of Japan},

volume = {91},

issue = {11},

pages = {113701},

abstract = {The magnetic structure of antiferromagnetic α-Mn has been unclarified for almost 70 years since its magnetism was discovered. We measured the zero-field nuclear magnetic resonance spectra of antiferromagnetic α-Mn to obtain further insight into magnetism below TN = 95 K. The site II spectra split into two sites with five subpeaks owing to quadrupole interaction, and this shows that the ordered moments at site II are slightly tilted from the [001] direction. The site III spectra revealed that this site splits into four sites below TN. These findings clearly demonstrate that the antiferromagnetic α-Mn symmetry is lower than previously considered.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{S.Kitagawa_PRB_2022,

title = {Enhancement of charge-neutral fermionic excitations near the spin-flop transition in the magnetic Kondo material YbIr3Si7},

author = {S Kitagawa and T Kobayashi and F Hori and K Ishida and A H Nevidomskyy and L Qian and E Morosan},

url = {https://doi.org/10.1103/PhysRevB.106.L100405

https://arxiv.org/abs/2209.10844},

doi = {10.1103/PhysRevB.106.L100405},

year  = {2022},

date = {2022-09-16},

urldate = {2022-09-16},

journal = {Physical Review B},

volume = {106},

pages = {L100405},

abstract = {The new Kondo material YbIr3Si7, similar to other Kondo insulators, has been reported to exhibit charge-neutral fermionic excitations through measurements of specific heat and thermal conductivity at low temperatures. 

We performed 29Si−NMR on YbIr3Si7 to investigate the magnetic response of charge-neutral fermions from a microscopic perspective. In low magnetic fields parallel to the c axis, a single NMR peak in the paramagnetic state splits into three peaks below TN. 

In contrast, only a slight shift of the single NMR peak was observed in high magnetic fields. This spectral change as a function of the c

-axis magnetic field is interpreted as a spin-flop transition, at which the magnetic moments oriented along the c axis antiferromagnetic (AFM-I) phase are rotated to the ab plane with a ferromagnetic component along the c-axis (AFM-II phase). In the vicinity of the spin-flop magnetic field 

HM, the nuclear spin-lattice relaxation rate 1/T1 was found to be proportional to temperature at low temperatures, indicating the existence of charge-neutral fermions. Furthermore, a peak of 1/T1 versus the c-axis magnetic field suggests that the charge-neutral fermions in YbIr3Si7 are closely related to its magnetic properties. Our findings shed light on the origin of charge-neutral fermions in insulators.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{T.Taniguchi_JPSJ_2022,

title = {139La-NMR Study of Spin Dynamics Coupled with Hole Mobility in T*-type La0.86Eu0.86Sr0.28CuO4−δ},

author = {T Taniguchi and S Kitagawa and K Ishida and S Asano and K Kudo and M Takahama and P Xie and T Noji and M Fujita},

url = {https://arxiv.org/abs/2110.12333},

doi = {10.7566/JPSJ.91.074710},

year  = {2022},

date = {2022-06-22},

urldate = {2022-06-22},

journal = {Journal of the Physical Society of Japan},

volume = {91},

issue = {07},

pages = {074710},

abstract = {In T*-type cuprate oxides with five-oxygen coordination, the relationship between spin correlations and doped carriers has not been well understood. To clarify this relationship, and the magnetic and superconducting (SC) properties of T*-type cuprate oxides, we performed 139La-nuclear magnetic resonance (NMR) and electrical resistivity measurements on as-sintered (AS) and oxidation-annealed (OA) polycrystalline T*-type La0.86Eu0.86S0.28CuO4 (LESCO) to investigate its magnetic and SC properties. Upon cooling, the NMR spectrum of AS LESCO broadened below 3 K, where the nuclear spin–lattice relaxation rate 1/T1 with respect to the temperature exhibited a maximum, indicating the appearance of static magnetism. The temperature dependence of 1/T1 between 3 and 20 K was similar to that of resistivity, which displays semiconducting behavior. Furthermore, it was found that the energy scales of the transport gap and spin dynamics were comparable. These results suggest a close connection between the mobility of the doped carriers and low-energy spin dynamics, as reported for lightly doped T-type La2−xSrxCuO4. In the OA SC sample, we confirmed the absence of a magnetic order and the linear relation between 1/T1 and T above 10 K. These results suggest that the magnetic state coupled with holes markedly changes to the weakly correlated metallic state by oxidation annealing in the T*-type LESCO with x = 0.28.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{K.Kinjo_Science_2022,

title = {Superconducting spin smecticity evidencing the Fulde-Ferrell-Larkin-Ovchinnikov state in Sr2RuO4},

author = {K Kinjo and M Manago and S Kitagawa and Z Q Mao and S Yonezawa and Y Maeno and K Ishida},

url = {https://www.science.org/doi/10.1126/science.abb0332},

doi = {10.1126/science.abb0332},

year  = {2022},

date = {2022-04-21},

urldate = {2022-04-21},

journal = {Science},

volume = {376},

issue = {6591},

pages = {397-400},

abstract = {Translational symmetry breaking is antagonistic to static fluidity but can be realized in superconductors, which host a quantum-mechanical coherent fluid formed by electron pairs. A peculiar example of such a state is the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, induced by a time-reversal symmetry–breaking magnetic field applied to spin-singlet superconductors. This state is intrinsically accompanied by the superconducting spin smecticity, spin density–modulated fluidity with spontaneous translational-symmetry breaking. Detection of such spin smecticity provides unambiguous evidence for the FFLO state, but its observation has been challenging. Here, we report the characteristic “double-horn” nuclear magnetic resonance spectrum in the layered superconductor Sr2RuO4 near its upper critical field, indicating the spatial sinusoidal modulation of spin density that is consistent with superconducting spin smecticity. Our work reveals that Sr2RuO4 provides a versatile platform for studying FFLO physics.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{K.Kinjo_PRB_2022,

title = {Drastic change in magnetic anisotropy of UTe2 under pressure revealed by 125Te -NMR},

author = {K Kinjo and H Fujibayashi and G Nakamine and S Kitagawa and K Ishida and Y Tokunaga and H Sakai and S Kambe and A Nakamura and Y Shimizu and Y Homma and D Li and F Honda and D Aoki},

url = {https://arxiv.org/abs/2203.12934},

doi = {10.1103/PhysRevB.105.L140502},

year  = {2022},

date = {2022-04-06},

urldate = {2022-04-06},

journal = {Physical Review B},

volume = {105},

issue = {14},

pages = {L140502},

abstract = {To investigate the normal-state magnetic properties of UTe2 under pressure, we perform 125Te nuclear magnetic resonance (NMR) measurements up to 2 GPa. Below 1.2 GPa, the b-axis NMR Knight shift shows a broad maximum at the so-called T_chimax on cooling, which is consistent with the magnetization measurement under pressure. T_chimax decreases with increasing pressure and disappears at the critical pressure Pc = 1.7 GPa, above which superconductivity is destroyed. This tendency is also observed in the temperature dependence of the nuclear spin-lattice relaxation rate 1/T1. At low pressures, 1/T1 shows a conventional Fermi-liquid behavior (1/T1T = constant) at low temperatures, indicating the formation of the heavy-fermion state. Above Pc, 1/T1T follows a 1/T behavior without any crossover to the heavy-fermion state down to the lowest temperature (~3 K). In addition, the NMR signals disappear below 3 K, due to the influence of the magnetically ordered moments. From the pressure dependence of the T_chimax and Knight shift, it was found that the Fermi surface character is abruptly changed at Pc, and that superconductivity is observed only in the heavy-fermion state.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Impurity-Robust Bulk Gapless Excitation in the Yb-Based Zigzag Chain Compound YbCuS2},

author = {F Hori and K Kinjo and S Kitagawa and K Ishida and Y Ohmagari and T Onimaru},

doi = {10.1088/1742-6596/2164/1/012027},

year  = {2022},

date = {2022-03-17},

journal = {J. Phys.: Conf. Ser.},

volume = {2164},

pages = {012027},

abstract = {We have performed 63Cu-nuclear quadrupole resonance (NQR) measurements using a lump sample of the Yb zigzag-chain compound YbCuS2 with a small surface area to investigate the sample dependence of low-temperature magnetic properties in YbCuS2 by comparing with the previous study with different powdered sample. The line width of NQR signals in the present lump sample is larger than that in the previous powdered sample. In addition, the transition temperature TN ∼ 0.92 K in the present lump sample is lower than that in the previous powdered sample (∼ 0.95 K). These results suggest that the quality of the present lump sample is worse than that of the previous powdered sample. However, the T-linear behavior of the nuclear spin-lattice relaxation rate 1/T1 was observed below 0.5 K and the value of 1/T1T in both samples is almost the same even though the sample quality and sample geometry are different. This suggests that T-linear behavior in 1/T1 arises from the impurity-robust bulk gapless excitation inherent in YbCuS2 rather than from sample issues such as the sample quality or geometry.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{H.Fujibayashi_JPSJ_2022,

title = {Superconducting Order Parameter in UTe2 Determined by Knight Shift Measurement},

author = {H Fujibayashi and G Nakamine and K Kinjo and S Kitagawa and K Ishida and Y Tokunaga and H Sakai and S Kambe and A Nakamura and Y Shimizu and Y Homma and D Li and F Honda and D Aoki},

url = {https://doi.org/10.7566/JPSJ.91.043705

https://arxiv.org/abs/2203.08598},

doi = {10.7566/JPSJ.91.043705},

year  = {2022},

date = {2022-03-11},

urldate = {2022-03-11},

journal = {Journal of the Physical Society of Japan},

volume = {91},

number = {04},

pages = {043705},

abstract = {This study investigates the spin susceptibility in U-based superconductor UTe2 in the superconducting (SC) state by using Knight shift measurements for a magnetic field H along the a axis, which is the magnetic easy axis of UTe2. 

Although a tiny anomaly ascribed to the SC diamagnetic effect was observed just below the SC transition temperature Tc, the a-axis Knight shift in the SC state shows no significant decrease, following the extrapolation from the normal-state temperature dependence. 

This indicates that the spin susceptibility is nearly unchanged below Tc. 

Considering the previous Knight shift results for H∥b and H∥c, the dominant SC state is determined to be B3u in the spin-triplet pairing, which is consistent with the spin anisotropy in the normal state. 

The present result shows that UTe2 is a spin-triplet superconductor with spin degrees of freedom.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{S.Kitagawa_JPSJ_2021,

title = {Two-Dimensional XY-Type Magnetic Properties of Locally Noncentrosymmetric Superconductor CeRh2As2},

author = {S Kitagawa and M Kibune and K Kinjo and M Manago and T Taniguchi and K Ishida and M Brando and E Hassinger and C Geibel and S Khim},

url = {https://doi.org/10.7566/JPSJ.91.043702

https://arxiv.org/abs/2203.03184},

doi = {10.7566/JPSJ.91.043702},

year  = {2022},

date = {2022-03-04},

journal = {Journal of the Physical Society of Japan},

volume = {91},

number = {04},

pages = {043702},

abstract = {We performed 75As-NMR measurements to investigate the normal-state magnetic properties of CeRh2As2, a recently-discovered heavy-fermion superconductor. 

The magnitude and temperature dependence of the Knight shift at the As(2) site indicate easy-plane-type magnetic anisotropy in CeRh2As2. 

With regard to spin fluctuations, the temperature dependence of the nuclear spin-lattice relaxation rate 1/T1 arising from the 4f electrons decreases from high-temperature constant behavior on cooling at ∼ 40~K, which is typical behavior of heavy-fermion systems. 

In addition, 1/T1 becomes constant at low temperatures, suggesting spatially two-dimensional antiferromagnetic fluctuations. Two-dimensional magnetic correlations in the real space are quite rare among heavy-fermion superconductors, and they may be a key factor in the unique superconducting multi-phase in CeRh2As2.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{S.Kitagawa_PRL_2022,

title = {Observation of Antiferromagnetic Order as Odd-Parity Multipoles inside the Superconducting Phase in  CeRh2As2},

author = {M Kibune and S Kitagawa and K Kinjo and S Ogata and M Manago and T Taniguchi and K Ishida and M Brando and E Hassinger and H Rosner and C Geibel and S Khim},

url = {https://arxiv.org/abs/2112.07081},

doi = {10.1103/PhysRevLett.128.057002},

year  = {2022},

date = {2022-02-03},

urldate = {2022-02-03},

journal = {Physical Review Letters},

volume = {128},

number = {05},

pages = {057002},

abstract = {Spatial inversion symmetry in crystal structures is closely related to the superconducting (SC) and magnetic properties of materials. Recently, several theoretical proposals that predict various interesting phenomena caused by the breaking of the local inversion symmetry have been presented. However, experimental validation has not yet progressed owing to the lack of model materials. Here we present evidence for antiferromagnetic (AFM) order in CeRh2As2 (SC transition temperature TSC ∼ 0.37 K ), wherein the Ce site breaks the local inversion symmetry. The evidence is based on the observation of different extents of broadening of the nuclear quadrupole resonance spectrum at two crystallographically inequivalent As sites. This AFM ordering breaks the inversion symmetry of this system, resulting in the activation of an odd-parity magnetic multipole. Moreover, the onset of antiferromagnetism TN within an SC phase, with TN < TSC , is quite unusual in systems wherein superconductivity coexists or competes with magnetism. Our observations show that CeRh2As2 is a promising system to study how the absence of local inversion symmetry induces or influences unconventional magnetic and SC states, as well as their interaction.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Y.Tokunaga_JPSJ_2022,

title = {Slow Electronic Dynamics in the Paramagnetic State of UTe2},

author = {Y Tokunaga and H Sakai and S Kambe and Y Haga and Y Tokiwa and P Opletal and H Fujibayashi and K Kinjo and S Kitagawa and K Ishida and A Nakamura and Y Shimizu and Y Homma and D Li and F Honda and D Aoki},

url = {https://arxiv.org/abs/2201.07455},

doi = {10.7566/JPSJ.91.023707},

year  = {2022},

date = {2022-01-27},

urldate = {2022-01-27},

journal = {Journal of the Physical Society of Japan},

volume = {91},

issue = {02},

pages = {023707},

abstract = {125Te NMR experiments in field (H) applied along the easy magnetization axis (the a-axis) revealed slow electronic dynamics developing in the paramagnetic state of UTe2. The observed slow fluctuations are concerned with a successive growth of long-range electronic correlations below 30–40 K, where the spin susceptibility along the hard magnetization axis (the b-axis) shows a broad maximum. The experiments also imply that tiny amounts of disorder or defects locally disturb the long-range electronic correlations and develop an inhomogeneous electronic state at low temperatures, leading to a low temperature upturn observed in the bulk-susceptibility in H || a. We suggest that UTe2 would be located on the paramagnetic side near an electronic phase boundary, where either magnetic or Fermi-surface instability would be the origin of the characteristic fluctuations.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Y.Sato_NComm_2022,

title = {Charge neutral fermions and magnetic field driven instability in insulating YbIr3Si7},

author = {Y Sato and S Suetsugu and T Tominaga and Y Kasahara and S Kasahara and T Kobayashi and S Kitagawa and K Ishida and R Peters and T Shibauchi and A H Nevidomskyy and L Qian and J M Moya and E Morosan and Y Matsuda},

url = {https://arxiv.org/abs/2103.13718},

doi = {10.1038/s41467-021-27541-9},

year  = {2022},

date = {2022-01-19},

urldate = {2022-01-19},

journal = {Nature Communications},

volume = {13},

pages = {394},

abstract = {Kondo lattice materials, where localized magnetic moments couple to itinerant electrons, provide a very rich backdrop for strong electron correlations. They are known to realize many exotic phenomena, with a dramatic example being recent observations of quantum oscillations and metallic thermal conduction in insulators, implying the emergence of enigmatic charge-neutral fermions. Here, we show that thermal conductivity and specific heat measurements in insulating YbIr3Si7 reveal emergent neutral excitations, whose properties are sensitively changed by a field-driven transition between two antiferromagnetic phases. In the low-field phase, a significant violation of the Wiedemann-Franz law demonstrates that YbIr3Si7 is a charge insulator but a thermal metal. In the high-field phase, thermal conductivity exhibits a sharp drop below 300 mK, indicating a transition from a thermal metal into an insulator/semimetal driven by the magnetic transition. These results suggest that spin degrees of freedom directly couple to the neutral fermions, whose emergent Fermi surface undergoes a field-driven instability at low temperatures.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{K.Ishida_PRB_2021,

title = {Pairing interaction in superconducting UCoGe tunable by magnetic field},

author = {K Ishida and S Matsuzaki and M Manago and T Hattori and S Kitagawa and M Hirata and T Sasaki and D Aoki},

url = {https://arxiv.org/abs/2109.14426},

doi = {10.1103/PhysRevB.104.144505},

year  = {2021},

date = {2021-10-18},

journal = {Physical Review B},

volume = {104},

number = {14},

pages = {144505},

abstract = {The mechanism of unconventional superconductivity, such as high-temperature-cuprate, Fe-based, and heavy-fermion superconductors, has been studied as a central issue in condensed-matter physics. Spin fluctuations, instead of phonons, are considered to be responsible for the formation of Cooper pairs, and many efforts have been made to confirm this mechanism experimentally. Although a qualitative consensus seems to have been obtained, experimental confirmation has not yet been achieved. This is owing to a lack of the quantitative comparison between theory and experiments. Here, we show a semiquantitative comparison between the superconducting-transition temperature ( T SC ) and spin fluctuations derived from the NMR experiment on the ferromagnetic (FM) superconductor UCoGe in which the FM fluctuations and superconductivity are tunable by external fields. The enhancement and abrupt suppression of T SC by applied fields, as well as the pressure variation of T SC around the FM criticality are well understood by the change in the FM fluctuations on the basis of the single-band spin-triplet theoretical formalism. The present comparisons strongly support the theoretical formalism of spin-fluctuation-mediated superconductivity, particularly in UCoGe.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{S.Kitagawa_PRB_2021,

title = {Peak in the superconducting transition temperature of the nonmagnetic topological line-nodal material CaSb_{2} under pressure},

author = {S Kitagawa and K Ishida and A Ikeda and M Kawaguchi and S Yonezawa and Y Maeno},

url = {https://arxiv.org/abs/2108.08992},

doi = {10.1103/PhysRevB.104.L060504},

year  = {2021},

date = {2021-08-17},

journal = {Physical Review B},

volume = {104},

number = {6},

pages = {L060504},

abstract = {Investigating the pressure dependence of the superconducting (SC) transition temperature Tc is crucial for understanding the SC mechanism. 

Herein, we report on the pressure dependence of Tc in the nonmagnetic topological line-nodal material CaSb2 based on measurements of electric resistance and alternating current magnetic susceptibility. 

Tc initially increases with increasing pressure and peaks at ~ 3.1 GPa. With a further increase in pressure, Tc decreases and finally becomes undetectable at 5.9 GPa. 

Because no signs of phase transition or Lifshitz transition are observed in the normal state, the peculiar peak structure of Tc suggests that CaSb2 has an unconventional SC character.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{H.Takahashi_JPSJ_2021,

title = {S-Wave Superconductivity in the Dirac Line-Nodal Material CaSb2},

author = {H Takahashi and S Kitagawa and K Ishida and M Kawaguchi and A Ikeda and S Yonezawa and Y Maeno},

url = {https://arxiv.org/abs/2105.13614},

doi = {10.7566/JPSJ.90.073702},

year  = {2021},

date = {2021-06-11},

urldate = {2021-06-11},

journal = {Journal of the Physical Society of Japan},

volume = {90},

pages = {073702},

abstract = {We performed 121/123Sb-nuclear quadrupole resonance (NQR) measurements on the superconducting (SC) line-nodal material CaSb2 in order to investigate electronic properties in the normal and SC states from a microscopic point of view. In the normal state, the nuclear spin–lattice relaxation rate 1/T1 for the Sb(1) site, which is responsible for the line-nodal parts, is approximately proportional to temperature, indicating the conventional Fermi liquid state. From comparison with band structure calculations, it is considered that the NQR properties related to the line-nodal character are hidden because the conventional behavior originating from Fermi-surface parts away from the nodes is dominant. In the SC state, a clear coherence peak just below the transition temperature and an exponential decrease at lower temperatures were observed in 1/T1. These results strongly suggest that conventional s-wave superconductivity with a full gap is realized in CaSb2.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{G.Nakamine_JPSJ_2021,

title = {Inhomogeneous Superconducting State Probed by 125Te NMR on UTe2},

author = {G Nakamine and K Kinjo and S Kitagawa and K Ishida and Y Tokunaga and H Sakai and S Kambe and A Nakamura and Y Shimizu and Y Homma and D Li and F Honda and D Aoki},

url = {https://arxiv.org/abs/2105.11823},

doi = {10.7566/JPSJ.90.064709},

year  = {2021},

date = {2021-05-25},

journal = {Journal of the Physical Society of Japan},

volume = {90},

number = {064709},

pages = {7},

abstract = {UTe2 is a recently discovered promising candidate for a spin-triplet superconductor. In contrast to conventional spin-singlet superconductivity, spin-triplet superconductivity possesses spin and angular momentum degrees of freedom. To detect these degrees of freedom and obtain the solid evidence of spin-triplet superconductivity in UTe2, we have performed 125Te-NMR measurements, which are sensitive to the local spin susceptibility at a nuclear site. We previously reported that the shoulder signal appears in NMR spectra below the superconducting (SC) transition temperature Tc in H || b, and a slight decrease in the Knight shift along the b and c axes (Kb and Kc, respectively) below Tc at a low magnetic field H. Although the decrease in Kc vanished above 5.5 T, the decrease in Kb was independent of H up to 6.5 T. To clarify the origin of the shoulder signal and the trace of the decrease in Kb, we compared the 125Te-NMR spectra obtained when H || b and H || c and measured the 125Te-NMR spectra for H || b up to 14.5 T. The intensity of the shoulder signal observed for H || b has a maximum at ∼6 T and vanishes above 10 T, although the superconductivity is confirmed by the χAC measurements, which can survive up to 14.5 T (maximum H in the present measurement). Moreover, the decrease in Kb in the SC state starts to be small around 7 T and almost zero at 12.5 T. This indicates that the SC spin state gradually changes with the application of H. Meanwhile, in H || c, an unexpected broadening without the shoulder signals was observed below Tc at 1 T, and this broadening was quickly suppressed with increasing H. We construct the H–T phase diagram for H || b and H || c based on the NMR measurements and discuss possible SC states with the theoretical consideration. We suggest that the inhomogeneous SC state characterized by the broadening of the NMR spectrum originates from the spin degrees of freedom.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{G.Nakamine_PRB_2021,

title = {Anisotropic response of spin susceptibility in the superconducting state of  UTe2  probed with  125Te − NMR  measurement},

author = {G Nakamine and K Kinjo and S Kitagawa and K Ishida and Y Tokunaga and H Sakai and S Kambe and A Nakamura and Y Shimizu and Y Homma and D Li and F Honda and D Aoki},

url = {https://arxiv.org/abs/2103.02876},

doi = {10.1103/PhysRevB.103.L100503},

year  = {2021},

date = {2021-03-17},

journal = {Physical Review B},

volume = {103},

number = {10},

pages = {L100503},

abstract = {To investigate spin susceptibility in a superconducting (SC) state, we measured the 125 Te -NMR Knight shifts at magnetic fields ( H ) up to 6.5 T along the b and c axes of single-crystal UTe 2 , a promising candidate for a spin-triplet superconductor. In the SC state, the Knight shifts along the b and c axes ( K b and K c , respectively) decreased slightly, and the decrease in K b was almost constant up to 6.5 T. The reduction in K c decreased with increasing H , and K c was unchanged through the SC transition temperature at 5.5 T, excluding the possibility of spin-singlet pairing. Our results indicate that spin susceptibilities along the b and c axes slightly decrease in the SC state in low H , and the H response of SC spin susceptibility is anisotropic on the b c plane. We discuss the possible d -vector state within the spin-triplet scenario and suggest that the dominant d -vector component for the case of H ∥ b changes above 13 T, where T c increases with increasing H .},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Yamamoto_NC_2020,

title = {Strain-induced creation and switching of anion vacancy layers in perovskite oxynitrides},

author = {T Yamamoto and A Chikamatsu and S Kitagawa and N Izumo and S Yamashita and H Takatsu and M Ochi and T Maruyama and M Namba and W Sun and T Nakashima and F Takeiri and K Fujii and M Yashima and Y Sugisawa and M Sano and Y Hirose and D Sekiba and C M Brown and T Honda and K Ikeda and T Otomo and K Kuroki and K Ishida and T Mori and K Kimoto and T Hasegawa and H Kageyama},

doi = {10.1038/s41467-020-19217-7},

year  = {2020},

date = {2020-11-23},

journal = {Nature Communications},

volume = {11},

pages = {5923},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Okuno_JPSJ_2020,

title = {Magnetic-Field Dependence of Novel Gap Behavior Related to the Quantum-Size Effect},

author = {T Okuno and Y Kinoshita and S Matsuzaki and S Kitagawa and K Ishida and M Hirata and T Sasaki and K Kusada and H Kitagawa},

url = {https://arxiv.org/abs/2007.07688},

doi = {10.7566/JPSJ.89.095002},

year  = {2020},

date = {2020-08-03},

journal = {Journal of the Physical Society of Japan},

volume = {89},

number = {9},

pages = {095002},

abstract = {195Pt-NMR measurements of Pt nanoparticles with a mean diameter of 4.0 nm were performed in a high magnetic field of approximately μ0H = 23.3 T to investigate the low-temperature electronic state of the nanoparticles. The characteristic temperature T*, below which the nuclear spin-lattice relaxation rate 1/T1 deviates from the relaxation rate of the bulk, shows a magnetic-field dependence. This dependence supports the theoretical prediction of the appearance of discrete energy levels.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{S_Kitagawa2020,

title = {Variation in Superconducting Symmetry Against Pressure on Noncentrosymmetric Superconductor Cd_{2}Re_{2}O_{7} Revealed by ^{185/187}Re Nuclear Quadrupole Resonance},

author = {S Kitagawa and K Ishida and T C Kobayashi and Y Matsubayashi and D Hirai and Z Hiroi},

url = {https://arxiv.org/abs/2004.00919},

doi = {10.7566/JPSJ.89.053701},

year  = {2020},

date = {2020-04-02},

journal = {Journal of the Physical Society of Japan},

volume = {89},

number = {5},

pages = {053701},

abstract = {We performed 185/187Re nuclear quadrupole resonance (NQR) measurements under pressure to investigate the superconducting properties of noncentrosymmetric superconductor Cd2Re2O7 under various crystal structures. The pressure dependence of superconducting transition temperature Tc determined through ac susceptibility measurements is consistent with the results of previous resistivity measurements [Kobayashi et al., J. Phys. Soc. Jpn. 80, 023715 (2011)]. Below 2.2 GPa, in the nuclear spin–lattice relaxation rate 1/T1, a clear coherence peak was observed just below Tc, indicating conventional s-wave superconductivity. In contrast, the coherence peak disappears at 3.1 GPa, suggesting a change in superconducting symmetry to the p-wave dominant state against pressure.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Okuno2020,

title = {NMR-based gap behavior related to the quantum size effect},

author = {T Okuno and M Manago and S Kitagawa and K Ishida and K Kusada and H Kitagawa},

url = {https://arxiv.org/abs/2003.07585},

doi = {10.1103/PhysRevB.101.121406},

year  = {2020},

date = {2020-03-30},

journal = {Physical Review B},

volume = {101},

number = {12},

abstract = {We conducted Pt195-NMR measurements on various-diameter Pt nanoparticles coated with polyvinylpyrrolidone in order to detect the quantum size effect and the discrete energy levels in the electron density of states, both of which were predicted by Kubo more than 50 years ago. We succeeded in separating the signals arising from the surface and interior regions and found that the nuclear spin-lattice relaxation rates in both regions show the metallic behavior at high temperatures. Surprisingly, the magnetic fluctuations in both regions exhibited anomalous behavior below the same temperature T∗, which points to a clear size dependence and is well scaled with δKubo. These results suggest that a size-tunable metal-insulator transition occurs in the Pt nanoparticles as a result of the quantum size effect. © 2020 American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Manago2019c,

title = {Superconductivity at the pressure-induced ferromagnetic critical region in UCoGe},

author = {M Manago and S Kitagawa and K Ishida and K Deguchi and N K Sato and T Yamamura},

doi = {10.7566/JPSJ.88.113704},

year  = {2019},

date = {2019-10-23},

journal = {Journal of the Physical Society of Japan},

volume = {88},

number = {11},

abstract = {The phase separation of the ferromagnetic (FM) and paramagnetic (PM) phases in the superconducting (SC) state of UCoGe at the FM critical region was investigated using 59Co nuclear quadrupole resonance (NQR) technique by taking advantage of its site-selective feature. The NQR measurements revealed that the first-order quantum phase transition occurs between the FM and the PM states. The nuclear spin–lattice relaxation rate 1=T1 exhibited a clear drop at the SC state in the PM phase, whereas it was not detected in the FM phase, which indicates that the superconductivity in the FM phase becomes weaker at the FM critical region due to the presence of the PM SC state. This result suggests that the SC condensation energy of the PM SC state is equal or larger than that of the FM SC state in this region. The pressure–temperature phase diagram of UCoGe was modified by taking the results from this study into account. ©2019 The Physical Society of Japan},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Nakamine2019b,

title = {Superconducting Properties of Heavy Fermion UTe_{2} Revealed by ^{125}Te-nuclear Magnetic Resonance},

author = {G Nakamine and S Kitagawa and K Ishida and Y Tokunaga and H Sakai and S Kambe and A Nakamura and Y Shimizu and Y Homma and D Li and F Honda and D Aoki},

doi = {10.7566/JPSJ.88.113703},

year  = {2019},

date = {2019-10-17},

journal = {Journal of the Physical Society of Japan},

volume = {88},

number = {11},

abstract = {We have performed the 125Te-nuclear magnetic resonance (NMR) measurement in the field along the b axis on the newly discovered superconductor UTe2, which is a candidate of a spin-triplet superconductor. The nuclear spin–lattice relaxation rate divided by temperature 1=T1T abruptly decreases below a superconducting (SC) transition temperature Tc without showing a coherence peak, indicative of UTe2 being an unconventional superconductor. It was found that the temperature dependence of 1=T1T in the SC state cannot be understood by a single SC gap behavior but can be explained by a two SC gap model. The Knight shift, proportional to the spin susceptibility, decreases below Tc, but the magnitude of the decrease is much smaller than the decrease expected in the spin-singlet pairing. Rather, the small Knight-shift decrease as well as the absence of the Pauli-depairing effect can be interpreted by the spin triplet scenario. © 2019 Society The Author(s) of Japan.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kitagawa2019,

title = {Universal relationship between low-energy antiferromagnetic fluctuations and superconductivity in BaFe_{2}(As_{1-x}P_{x})_{2}},

author = {S Kitagawa and T Kawamura and K Ishida and Y Mizukami and S Kasahara and T Shibauchi and T Terashima and Y Matsuda},

doi = {10.1103/PhysRevB.100.060503},

year  = {2019},

date = {2019-08-12},

journal = {Physical Review B},

volume = {100},

number = {6},

abstract = {To identify the key parameter for optimal superconductivity in iron pnictides, we measured the P31-NMR relaxation rate on BaFe2(As1-xPx)2(x=0.22 and 0.28) under pressure and compared the effects of chemical substitution and physical pressure. For x=0.22, structural and antiferromagnetic (AFM) transition temperatures both show minimal changes with pressure up to 2.4 GPa, whereas the superconducting transition temperature Tc increases to twice its former value. In contrast, for x=0.28 near the AFM quantum critical point (QCP), the structural phase transition is quickly suppressed by pressure and Tc reaches a maximum. The analysis of the temperature-dependent nuclear relaxation rate indicates that these contrasting behaviors can be quantitatively explained by a single curve of the Tc dome as a function of Weiss temperature θ, which measures the distance to the QCP. Moreover, the Tc-θ curve under pressure precisely coincides with that with a chemical substitution, which is indicative of the existence of a universal relationship between low-energy AFM fluctuations and superconductivity on BaFe2(As1-xPx)2. © 2019 American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Asano2019,

title = {Oxidation annealing effects on the spin-glass-like magnetism and appearance of superconductivity in T*-type La_{1−x/2}Eu_{1−x/2}Sr_{x}CuO_{4} (0.14 ≤ x ≤ 0.28)},

author = {S Asano and K M Suzuki and K Kudo and I Watanabe and A Koda and R Kadono and T Noji and Y Koike and T Taniguchi and S Kitagawa and K Ishida and M Fujita},

doi = {10.7566/JPSJ.88.084709},

year  = {2019},

date = {2019-07-23},

journal = {Journal of the Physical Society of Japan},

volume = {88},

number = {8},

abstract = {We investigated the magnetism and superconductivity in as-sintered (AS) and oxidation annealed (OA) T+-type La1−x=2Eu1−x=2SrxCuO4 (LESCO) with 0.14 ≤ x ≤ 0.28 by the first comprehensive muon spin rotation=relaxation (μSR), magnetic susceptibility, and electrical resistivity measurements. In OA superconducting samples, no evidence of magnetic order was observed, whereas AS semiconducting samples exhibited evidence of a disordered magnetic state in the measured temperature range between ∼4 and ∼8 K. Therefore, the ground state in LESCO drastically varies with oxidation annealing and the magnetic phase competitively exists with the superconducting (SC) phase. The magnetic phase in the AS LESCO is quite robust against Sr doping, while the SC phase degrades with increasing x. A monotonous decrease of the SC transition temperature from 24.5 K in x = 0.14 to 9.0 K in x = 0.28 suggests the disappearance of the SC phase at x ∼ 0.34. Furthermore, we clarified the simultaneous development of (quasi) static magnetism and the electrical resistivity at a low temperature in AS samples, suggesting the inducement of magnetism by the suppression of carrier mobility. The variation in magnetism due to annealing is discussed from a viewpoint of structural defects, which was previously reported from neutron diffraction measurements. ©2019 The Physical Society of Japan},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Manago2019,

title = {Spin-triplet superconductivity in the paramagnetic UCoGe under pressure studied by ^{59}Co NMR},

author = {M Manago and S Kitagawa and K Ishida and K Deguchi and N K Sato and T Yamamura},

doi = {10.1103/PhysRevB.100.035203},

year  = {2019},

date = {2019-07-12},

journal = {Physical Review B},

volume = {100},

number = {3},

abstract = {A Co59 nuclear magnetic resonance (NMR) measurement was performed on the single-crystalline ferromagnetic (FM) superconductor UCoGe under a pressure of 1.09 GPa, where the FM state is suppressed and superconductivity occurs in the paramagnetic (PM) state, to study the superconducting (SC) state in the PM state. Co59-NMR spectra became broader but hardly shifted across the SC transition temperature. The Knight-shift change determined from fitting the spectral peak with a Gaussian was much smaller than the spin part of the Knight shift; this is in good agreement with the spin-triplet pairing suggested from the large upper critical field. The spectrum broadening in the SC state cannot be attributed to the SC diamagnetic effect but is related to the properties of spin-triplet superconductivity. The origins of the broadening are discussed herein. © 2019 American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Tokunaga2019,

title = {^{125}Te-NMR study on a single crystal of heavy fermion superconductor UTe_{2}},

author = {Y Tokunaga and H Sakai and S Kambe and T Hattori and N Higa and G Nakamine and S Kitagawa and K Ishida and A Nakamura and Y Shimizu and Y Homma and D Li and F Honda and D Aoki},

doi = {10.7566/JPSJ.88.073701},

year  = {2019},

date = {2019-06-24},

journal = {Journal of the Physical Society of Japan},

volume = {88},

number = {7},

abstract = {We report 125Te NMR studies on a newly discovered heavy fermion superconductor UTe2. Using a single crystal, we have measured the 125Te NMR Knight shift K and spin-lattice relaxation rate 1=T1 for fields along the three orthorhombic crystal axes. The data confirm a moderate Ising anisotropy for both the static (K) and dynamical susceptibilities (1=T1) in the paramagnetic state above about 20 K. Around 20 K, however, we have observed a sudden loss of NMR spin-echo signal due to sudden enhancement of the NMR spin-spin relaxation rate 1=T2, when the field is applied along the easy axis of magnetization (= a-axis). This behavior suggests the development of longitudinal magnetic fluctuations along the a-axis at very low frequencies below 20 K. © 2019 The Physical Society of Japan},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kinjo2019,

title = {Magnetic field effect on s-wave superconductor LaRu_{4}P_{12} Studied by ^{31}P-NMR},

author = {K Kinjo and S Kitagawa and Y Nakai and K Ishida and H Sugawara and H Sato},

doi = {10.7566/JPSJ.88.065002},

year  = {2019},

date = {2019-05-30},

journal = {Journal of the Physical Society of Japan},

volume = {88},

number = {6},

abstract = {We have performed 31P-NMR measurements on the s-wave superconductor LaRu4P12 to investigate the magnetic field effect of the nuclear spin–lattice relaxation rate 1=T1 on a conventional full-gap superconductor. With increasing magnetic field, the Hebel–Slichter peak immediately below Tc in 1=T1 was suppressed, and the magnetic field dependence of 1=T1 at 0.8K, well below Tc, was proportional to H2. These behaviors can be fully understood by the orbital pair-breaking effect in a single-band s-wave superconductor. © 2019 The Physical Society of Japan},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Nakamine2019,

title = {Modification of magnetic fluctuations by interfacial interactions in artificially engineered heavy-fermion superlattices},

author = {G Nakamine and T Yamanaka and S Kitagawa and M Naritsuka and T Ishii and T Shibauchi and T Terashima and Y Kasahara and Y Matsuda and K Ishida},

doi = {10.1103/PhysRevB.99.081115},

year  = {2019},

date = {2019-02-19},

journal = {Physical Review B},

volume = {99},

number = {8},

abstract = {Recent progress in the fabrication techniques of superlattices (SLs) has made it possible to sandwich several-layer-thick block layers (BLs) of heavy-fermion superconductor CeCoIn5 between conventional-metal YbCoIn5 BLs or spin-density-wave-metal CeRhIn5 BLs of a similar thickness. However, the magnetic state in each BL, particularly at the interface, is not yet understood, as experimental techniques applicable to the SL system are limited. Here, we report measurements of Co59 nuclear magnetic resonance, which is a microscopic probe of the magnetic properties inside the target BLs. In the CeCoIn5/YbCoIn5 SL, the low-temperature magnetic fluctuations of the CeCoIn5 BL are weakened as expected from the Rashba spin-orbit effect. However, in the CeCoIn5/CeRhIn5 SL, the fluctuations show an anomalous enhancement below 6 K, highlighting the importance of the magnetic proximity effect occurring near a magnetic-ordering temperature TN∼3 K of the CeRhIn5 BL. We suggest that the magnetic properties of the BLs can be altered by the interfacial interaction, which is an alternative route to modify the magnetic properties. © 2019 American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Aoki2019,

title = {Review of U-based ferromagnetic superconductors: Comparison between UGE_{2}, URHGE, and UCOGE},

author = {D Aoki and K Ishida and J Flouquet},

doi = {10.7566/JPSJ.88.022001},

year  = {2019},

date = {2019-01-29},

journal = {Journal of the Physical Society of Japan},

volume = {88},

number = {2},

abstract = {The discovery in 2000 that the ferromagnetic (FM) compound UGe2 (TCurie = 52 K at ambient pressure) becomes superconducting under a pressure of P = 1.1 GPa until it enters the paramagnetic (PM) phase above Pc = 1.6 GPa was a surprise, despite the fact that such a possibility was emphasized in theory four decades ago. Successive searches for new materials (URhGe and UCoGe) led to the discovery of the coexistence of superconductivity (SC) and ferromagnetism at ambient pressure. Furthermore in UCoGe, it was found that SC survives in the PM regime from Pc = 1.1 to 4 GPa. The novelty is that SC also emerges deep inside the FM regime but with strong FM fluctuations. Focus has been on low-temperature experiments under extreme conditions of magnetic field (H), pressure, and uniaxial stress. NQR and NMR experiments are unique tools to understand the interplay between the spin dynamics and the Cooper pairing. We choose to present the SC properties from the knowledge of quasiparticle dressing in the normal phase (renormalized band mass, mB plus the extra dressing originating from FM fluctuations, m++). In UGe2, strong interplay exists between Fermi surface (FS) reconstructions in the cascade of different FM and PM ground states and their magnetic fluctuations. Similar phenomena occur in URhGe and UCoGe but, at first glance, the SC seems to be driven by the FM fluctuations. The weakness of the FM interaction in these two compounds gives the opportunity to observe singular features in magnetic field scans depending on their field orientation with respect to the FM sublattice magnetization (M0). We will show that for UCoGe, which has the smallest ordered moment, a longitudinal field scan (H ∥ M0) leads to a drastic decrease in the FM fluctuations with direct consequences on SC properties such as the upward curvature of the upper critical field. A transverse field scan (H M0) leads to suppression of the Curie temperature, TCurie; the consequence is a boost in FM fluctuations, which leads to a reinforcement of SC. Contrary to the two examples of Ising FM UGe2 and UCoGe, the singularity in URhGe is the weakness of the magnetocrystalline term between the choice of ferromagnetism along the c- or b-axis; the most noteworthy feature is the detection of reentrant SC on each side of the H switch at HR from the c easy axis of magnetization to the b-axis. All the experimental results give evidence that the SC in these three materials originates from the FM fluctuations, which are amplitude modes of magnetic excitations in the FM state. Spin-triplet pairing has been anticipated in the FM superconductors and was actually observed by Knight-shift measurements in the SC state of UCoGe. Their fascinating ð P; T; H Þ phase diagrams are now well established. Of course, a new generation of experiments will elucidate subtle effects by obtaining their SC order parameters. While the FSs of UGe2 have been experimentally well determined, those of URhGe and UCoGe have been poorly determined, and thus a quantitative comparison with band structure calculations cannot be achieved. Up to now, Angle-resolved photoemission spectroscopy (ARPES) measurements have only given the flavor of the electronic bands at the Fermi level. Discussion is presented on how different theoretical approaches can describe the various phenomena discovered by experimentalists. Following the new hot subject of topological superconductors, proposals have been made for UCoGe, which is a great challenge for ambitious researchers!. © 2019 The Physical Society of Japan},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Manago2019b,

title = {Enhancement of superconductivity by pressure-induced critical ferromagnetic fluctuations in UCoGe},

author = {M Manago and S Kitagawa and K Ishida and K Deguchi and N K Sato and T Yamamura},

doi = {10.1103/PhysRevB.99.020506},

year  = {2019},

date = {2019-01-24},

journal = {Physical Review B},

volume = {99},

number = {2},

abstract = {A Co59 nuclear quadrupole resonance (NQR) was performed on a single-crystalline ferromagnetic (FM) superconductor UCoGe under pressure. The FM phase vanished at a critical pressure Pc, and the NQR spectrum just below Pc showed phase separation of the FM and paramagnetic (PM) phases below Curie temperature TCurie, suggesting first-order FM quantum phase transition (QPT). We found that the internal field was absent above Pc, but the superconductivity is almost unchanged. This result suggests the existence of the nonunitary to unitary transition of the superconductivity around Pc. Nuclear spin-lattice relaxation rate 1/T1 showed the FM critical fluctuations around Pc, which persist above Pc and are clearly related to superconductivity in the PM phase. This FM QPT is understood to be a weak first order with critical fluctuations. 1/T1 sharply decreased in the superconducting (SC) state above Pc with a single component, in contrast to the two-component 1/T1 in the FM SC state, indicating that the inhomogeneous SC state is a characteristic feature of the FM SC state in UCoGe. © 2019 American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kitagawa2018,

title = {Normal-state properties of the antiperovskite oxide Sr3-xSnO revealed by Sn 119 -NMR},

author = {S Kitagawa and K Ishida and M Oudah and J N Hausmann and A Ikeda and S Yonezawa and Y Maeno},

url = {http://hdl.handle.net/2433/235675

https://arxiv.org/abs/1809.09275},

doi = {10.1103/PhysRevB.98.100503},

year  = {2018},

date = {2018-09-21},

journal = {Physical Review B},

volume = {98},

number = {10},

pages = {100503(R)},

abstract = {We have performed Sn119-NMR measurements on the antiperovskite oxide superconductor Sr3-xSnO to investigate how its normal state changes with the Sr deficiency. A two-peak structure was observed in the NMR spectra of all the measured samples. This suggests that the phase separation tends to occur between the nearly stoichiometric and heavily Sr-deficient Sr3-xSnO phases. The measurement of the nuclear spin-lattice relaxation rate 1/T1 indicates that the Sr-deficient phase shows a conventional metallic behavior due to the heavy hole doping. In contrast, the nearly stoichiometric phase exhibits unusual temperature dependence of 1/T1, attributable to the presence of a Dirac-electron band. © 2018 American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kitagawa2018b,

title = {Nematic transition and highly two-dimensional superconductivity in BaTi2Bi2 O revealed by Bi 209 -nuclear magnetic resonance/nuclear quadrupole resonance measurements},

author = {S Kitagawa and K Ishida and W Ishii and T Yajima and Z Hiroi},

doi = {10.1103/PhysRevB.98.220507},

year  = {2018},

date = {2018-01-01},

journal = {Physical Review B},

volume = {98},

number = {22},

abstract = {In this Rapid Communication, a set of Bi209-nuclear magnetic resonance (NMR)/nuclear quadrupole resonance (NQR) measurements has been performed to investigate the physical properties of superconducting (SC) BaTi2Bi2O from a microscopic point of view. The NMR and NQR spectra at 5 K can be reproduced with a nonzero in-plane anisotropic parameter η, indicating the breaking of the in-plane fourfold symmetry at the Bi site without any magnetic order, i.e., "the electronic nematic state." In the SC state, the nuclear spin-lattice relaxation rate divided by temperature, 1/T1T, does not change even below Tc, while a clear SC transition was observed with a diamagnetic signal. This observation can be attributed to the strong two dimensionality in BaTi2Bi2O. Comparing the NMR/NQR results among BaTi2Pn2O (Pn=As, Sb, and Bi), it was found that the normal and SC properties of BaTi2Bi2O were considerably different from those of BaTi2Sb2O and BaTi2As2O, which might explain the two-dome structure of Tc in this system. © 2018 American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Smidman20182930,

title = {Interplay between unconventional superconductivity and heavy-fermion quantum criticality: CeCu2Si2 versus YbRh2Si2},

author = {M Smidman and O Stockert and J Arndt and G M Pang and L Jiao and H Q Yuan and H A Vieyra and S Kitagawa and K Ishida and K Fujiwara and T C Kobayashi and E Schuberth and M Tippmann and L Steinke and S Lausberg and A Steppke and M Brando and H Pfau and U Stockert and P Sun and S Friedemann and S Wirth and C Krellner and S Kirchner and E M Nica and R Yu and Q Si and F Steglich},

doi = {10.1080/14786435.2018.1511070},

year  = {2018},

date = {2018-01-01},

journal = {Philosophical Magazine},

volume = {98},

number = {32},

pages = {2930-2963},

abstract = {In this paper the low-temperature properties of two isostructural canonical heavy-fermion compounds are contrasted with regards to the interplay between antiferromagnetic (AF) quantum criticality and superconductivity. For CeCu2Si2, fully-gapped d-wave superconductivity forms in the vicinity of an itinerant three-dimensional heavy-fermion spin-density-wave (SDW) quantum critical point (QCP). Inelastic neutron scattering results highlight that both quantum critical SDW fluctuations as well as Mott-type fluctuations of local magnetic moments contribute to the formation of Cooper pairs in CeCu2Si2. In YbRh2Si2, superconductivity appears to be suppressed at T ⪆ 10 mK by AF order (TN = 70 mK). Ultra-low temperature measurements reveal a hybrid order between nuclear and 4f-electronic spins, which is dominated by the Yb-derived nuclear spins, to develop at TA slightly above 2 mK. The hybrid order turns out to strongly compete with the primary 4f-electronic order and to push the material towards its QCP. Apparently, this paves the way for heavy-fermion superconductivity to form at Tc = 2 mK. Like the pressure – induced QCP in CeRhIn5, the magnetic field – induced one in YbRh2Si2 is of the local Kondo-destroying variety which corresponds to a Mott-type transition at zero temperature. Therefore, these materials form the link between the large family of about fifty low-T unconventional heavy – fermion superconductors and other families of unconventional superconductors with higher Tcs, notably the doped Mott insulators of the cuprates, organic charge-transfer salts and some of the Fe-based superconductors. Our study suggests that heavy-fermion superconductivity near an AF QCP is a robust phenomenon. © 2018, © 2018 Informa UK Limited, trading as Taylor & Francis Group.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kitagawa2018c,

title = {Evidence for the Presence of the Fulde-Ferrell-Larkin-Ovchinnikov State in CeCu2Si2 Revealed Using Cu 63 NMR},

author = {S Kitagawa and G Nakamine and K Ishida and H S Jeevan and C Geibel and F Steglich},

doi = {10.1103/PhysRevLett.121.157004},

year  = {2018},

date = {2018-01-01},

journal = {Physical Review Letters},

volume = {121},

number = {15},

abstract = {Nuclear magnetic resonance measurements were performed on CeCu2Si2 in the presence of a magnetic field close to the upper critical field μ0Hc2 in order to investigate its superconducting (SC) properties near pair-breaking fields. In lower fields, the Knight shift and nuclear spin-lattice relaxation rate divided by temperature 1/T1T abruptly decreased below the SC transition temperature Tc(H), a phenomenon understood within the framework of conventional spin-singlet superconductivity. In contrast, 1/T1T was enhanced just below Tc(H) and exhibited a broad maximum when magnetic fields close to μ0Hc2(0) were applied parallel or perpendicular to the c axis; although the Knight shift decreased just below Tc(H). This enhancement of 1/T1T, which was recently observed in the organic superconductor κ-(BEDT-TTF)2Cu(NCS)2, suggests the presence of high-density Andreev bound states in the inhomogeneous SC region, a hallmark of the Fulde-Ferrell-Larkin-Ovchinnikov phase. © 2018 American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Yamamoto20181566,

title = {Selective Hydride Occupation in BaVO3-xHx (0.3 ≤ × ≤ 0.8) with Faceand Corner-Shared Octahedra},

author = {T Yamamoto and K Shitara and S Kitagawa and A Kuwabara and M Kuroe and K Ishida and M Ochi and K Kuroki and K Fujii and M Yashima and C M Brown and H Takatsu and C Tassel and H Kageyama},

doi = {10.1021/acs.chemmater.7b04571},

year  = {2018},

date = {2018-01-01},

journal = {Chemistry of Materials},

volume = {30},

number = {5},

pages = {1566-1574},

abstract = {A growing number of transition metal oxyhydrides have recently been reported, but they are all confined to perovskite-related structures with corner-shared octahedra. Using high pressure synthesis, we have obtained vanadium oxyhydrides BaVO3-xHx (0.3 ≤ x ≤ 0.8) with a 6H-type hexagonal layer structure consisting of face-shared as well as corner-shared octahedra. Synchrotron X-ray and neutron diffraction measurements revealed that, in BaVO2.7H0.3, H- anions are located selectively at the face-shared sites, as supported by DFT calculations, while BaVO2.2H0.8 contains H- anions at both sites though the face-shared preference is partially retained. The selective hydride occupation for BaVO2.7H0.3 appears to suppress electron hopping along the c axis, making this material a quasi-two-dimensional metal characterized by anomalous temperature dependence of the electrical resistivity and strong antiferromagnetic fluctuations. In contrast, the anion disordered BaVO3-xHx in hexagonal (x ≈ 0.8) and cubic (x ≈ 0.9) forms exhibits a semiconducting behavior. This study offers a wide opportunity to develop transition metal oxyhydrides having various polyhedral linkages, along with site preference of H/O anions, aimed at finding interesting phenomena. © 2018 American Chemical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Manago2018,

title = {Single ferromagnetic fluctuations in UCoGe revealed by Ge 73 - And Co 59 -NMR studies},

author = {M Manago and K Ishida and D Aoki},

doi = {10.1103/PhysRevB.97.075130},

year  = {2018},

date = {2018-01-01},

journal = {Physical Review B},

volume = {97},

number = {7},

abstract = {Ge73 and Co59 nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements have been performed on a Ge73-enriched single-crystalline sample of the ferromagnetic superconductor UCoGe in the paramagnetic state. The Ge73 NQR parameters deduced from NQR and NMR are close to those of another isostructural ferromagnetic superconductor URhGe. The Knight shifts of the Ge and Co sites are well scaled to each other when the magnetic field is parallel to the b or c axis. The hyperfine coupling constants of Ge are estimated to be close to those of Co. The large difference of spin susceptibilities between the a and b axes could lead to the different response of the superconductivity and ferromagnetism with the field parallel to these directions. The temperature dependence of the nuclear spin-lattice relaxation rates 1/T1 at the two sites is similar to each other above 5 K. These results indicate that the itinerant U-5f electrons are responsible for the ferromagnetism in this compound, consistent with previous studies. The similarities and differences in the three ferromagnetic superconductors are discussed. © 2018 American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kitagawa2018d,

title = {Spatially inhomogeneous superconducting state near Hc2 in UPd2Al3},

author = {S Kitagawa and R Takaki and M Manago and K Ishida and N K Sato},

doi = {10.7566/JPSJ.87.013701},

year  = {2018},

date = {2018-01-01},

journal = {Journal of the Physical Society of Japan},

volume = {87},

number = {1},

abstract = {We have performed 27Al-NMR measurements on single-crystalline UPd2Al3 with the field parallel to the c-axis to investigate the superconducting (SC) properties near the upper critical field of superconductivity Hc2. The broadening of the NMR linewidth below 14K indicates the appearance of the internal field at the Al site, which originates from the antiferromagnetically ordered moments of U 5 f electrons. In the SC state well below μ0Hc2 = 3.4 T, the broadening of the NMR linewidth due to the SC diamagnetism and a decrease in the Knight shift are observed, which are wellunderstood by the framework of spin-singlet superconductivity. In contrast, the Knight shift does not change below Tc(H), and the NMR spectrum is broadened symmetrically in the SC state in the field range of 3 T < μ0H < μ0Hc2. The unusual NMR spectrum near Hc2 suggests that a spatially inhomogeneous SC state such as the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state would be realized. © 2018 The Physical Society of Japan.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Shi2018,

title = {High-field superconductivity on iron chalcogenide FeSe},

author = {A Shi and S Kitagawa and K Ishida and A E Böhmer and C Meingast and T Wolf},

doi = {10.7566/JPSJ.87.065002},

year  = {2018},

date = {2018-01-01},

journal = {Journal of the Physical Society of Japan},

volume = {87},

number = {6},

abstract = {We have performed ac-susceptibility and77Se-NMR measurements on single-crystal FeSe in the field range from 12.5 to 14.75T below 1.6K in order to investigate the superconducting properties of the B phase. Our results show that although superconductivity persists beyond the A–B transition line (H+), the broadening of the 77Se-NMR linewidth arising from the superconducting diamagnetic effect decreases at around H+, suggesting that superconducting character is changed at H+. © 2018 The Physical Society of Japan.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{H.Takahashi_PRBL_2024,

title = {Pressure evolution of the normal- and superconducting-state properties of the line-nodal material CaSb2 revealed by 123Sb nuclear quadrupole resonance},

author = {H Takahashi and S Kitagawa and K Ishida and A Ikeda and S R Saha and S Yonezawa and J Paglione and Y Maeno},

url = {https://arxiv.org/abs/2402.12783},

doi = {10.1103/PhysRevB.109.L100501},

year  = {2024},

date = {2024-03-04},

urldate = {2024-03-04},

journal = {Physical Review B},

volume = {109},

pages = {L100501},

abstract = {CaSb 2 is the Dirac line-nodal material that exhibits a superconducting (SC) transition at 1.7 K. In spite of its conventional SC state at ambient pressure, the transition temperature T c shows a peak structure against hydrostatic pressure. We performed ac magnetic susceptibility and 123 Sb nuclear quadrupole resonance (NQR) measurements on single-crystalline CaSb 2 under pressures up to 2.08 GPa. T c monotonically increased in this pressure region, which is consistent with a previous study. We observed continuous broadening of the NQR spectrum against pressure, which is a sign of unique compression behavior of the lattice. In the normal state, the nuclear spin-lattice relaxation rate 1 / T 1 is proportional to temperature in all pressure values, typical of a metal. However, 1 / T 1 T in the normal state is independent of pressure, indicating that the density of states at the Fermi energy N ( E F ) , which is one of the parameters governing T c , is insensitive to pressure. From these results, we conclude that N ( E F ) does not govern the origin of the enhancement in T c . This is unusual for a weak electron-phonon coupling superconductor. In the SC state, we revealed that the SC gap becomes larger and more isotropic under pressure.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{S.Kitagawa_PRB_2024,

title = {Breakdown of Kubo relation in Pt-Cu nanoparticles},

author = {S Kitagawa and Y Kinoshita and K Ishida and K Kusada and H Kitagawa},

doi = {10.1103/PhysRevB.109.L041408},

year  = {2024},

date = {2024-01-26},

urldate = {2024-01-26},

journal = {Physical Review B},

volume = {109},

pages = {L041408},

abstract = {Nanoparticles were predicted to exhibit unique physical properties due to quantum size effects, but their identification remains difficult.

According to Kubo's theory, the gap size is inversely correlated with both the density of states at the Fermi energy and the number of atoms in the particle.

Previously, we confirmed that the particle size and magnetic field dependence of NMR anomaly temperature is consistent with the estimated ``Kubo'' gap.

Here, we investigated the density-of-states dependence in the Pt$_{1-x}$Cu$_{x}$ nanoparticles.

While an enhancement of nuclear spin-lattice relaxation rate $1/T_1$ at low temperatures was clearly observed for the Pt-rich nanoparticles, such behavior was abruptly suppressed in the Cu-rich nanoparticles.

Furthermore, the NMR anomaly temperature is nearly unchanged with varying the density of states.

Our findings indicate that the quantum size effect contains more profound physics than just the ones predicted by Kubo.

},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Y.Tokunaga_PRL_2023,

title = {Longitudinal Spin Fluctuations Driving Field-Reinforced Superconductivity in  UTe2},

author = {Y Tokunaga and H Sakai and S Kambe and P Opletal and Y Tokiwa and Y Haga and S Kitagawa and K Ishida and D Aoki and G Knebel and G Lapertot and S Krämer and M. Horvatić},

url = {https://arxiv.org/abs/2307.10724},

doi = {10.1103/PhysRevLett.131.226503},

year  = {2023},

date = {2023-11-29},

urldate = {2023-11-29},

journal = {Physical Review Letters},

volume = {131},

pages = {226503},

abstract = {Our measurements of 125 Te NMR relaxations reveal an enhancement of electronic spin fluctuations above μ0H∗ ∼ 15 T, leading to their divergence in the vicinity of the metamagnetic transition at μ0Hm ≈ 35 T, below which field-reinforced superconductivity appears when a magnetic field(H) is applied along the crystallographic b axis. 

The NMR data evidence that these fluctuations are dominantly longitudinal, providing a key to understanding the peculiar superconducting phase diagram in H ∥ b, where such fluctuations enhance the pairing interactions.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Large perpendicular magnetic anisotropy induced by an inter-site charge transfer in strained EuVO2H films},

author = {M Namba and H Takatsu and R Mikita and Y Sijia and K Murayama and H-B Li and R Terada and C Tassel and H Ubukata and M Ochi and R S Puche and E P Latasa and N Ishimatsu and D Shiga and H Kumigashira and K Kinjo and S Kitagawa and K Ishida and T Terashima and K Fujita and T Mashiko and K Yanagisawa and K Kimoto and H Kageyama},

doi = {10.1021/jacs.3c04521},

year  = {2023},

date = {2023-09-28},

urldate = {2023-09-28},

journal = {Journal of the American Chemical Society　},

volume = {145},

pages = {21807},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Superconducting spin reorientation in spin-triplet multiple superconducting phases of UTe2},

author = {K Kinjo and H Fujibayashi and H Matsumura and F Hori and S Kitagawa and K Ishida and Y Tokunaga and H Sakai and S Kambe and A Nakamura and Y Shimizu and Y Homma and D Li and F Honda and D Aoki},

url = {https://arxiv.org/abs/2307.15784},

doi = {10.1126/sciadv.adg2736},

year  = {2023},

date = {2023-07-28},

urldate = {2023-07-28},

journal = {Science Advances},

volume = {9},

pages = {adg2736},

abstract = {Superconducting (SC) state has spin and orbital degrees of freedom, and spin-triplet superconductivity shows multiple SC phases because of the presence of these degrees of freedom. However, the observation of spin-direction rotation occurring inside the SC state (SC spin rotation) has hardly been reported. Uranium ditelluride, a recently found topological superconductor, exhibits various SC phases under pressure: SC state at ambient pressure (SC1), high-temperature SC state above 0.5 gigapascal (SC2), and low-temperature SC state above 0.5 gigapascal (SC3). We performed nuclear magnetic resonance (NMR) and ac susceptibility measurements on a single-crystal uranium ditelluride. The b axis spin susceptibility remains unchanged in SC2, unlike in SC1, and decreases below the SC2-SC3 transition with spin modulation. These unique properties in SC3 arise from the coexistence of two SC order parameters. Our NMR results confirm spin-triplet superconductivity with SC spin parallel to b axis in SC2 and unveil the remaining of spin degrees of freedom in SC uranium ditelluride.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Gapless fermionic excitation in the antiferromagnetic state of ytterbium zigzag chain},

author = {F Hori and K Kinjo and S Kitagawa and K Ishida and S Mizutani and R Yamamoto and Y Ohmagari and T Onimaru },

url = {https://arxiv.org/abs/2201.07563},

doi = {10.1038/s43246-023-00381-4},

year  = {2023},

date = {2023-07-22},

urldate = {2023-07-22},

journal = {Communications Materials},

volume = {4},

pages = {55},

abstract = {The emergence of charge-neutral fermionic excitations in magnetic systems is one of the unresolved issues in recent condensed matter physics. This type of excitations has been observed in various systems, such as low-dimensional quantum spin liquids, Kondo insulators, and antiferromagnetic insulators. Here, we report the presence of a pronounced gapless spin excitation in the low-temperature antiferromagnetic state of YbCuS2 semiconductor, where trivalent ytterbium atoms form a zigzag chain structure. We confirm the presence of this gapless excitations by a combination of experimental probes, namely 63/65Cu-nuclear magnetic resonance and nuclear quadrupole resonance, as well as specific heat measurements, revealing a linear low-temperature behavior of both the nuclear spin-lattice relaxation rate 1/T1 and the specific heat. This system provides a platform to investigate the origin of gapless excitations in spin chains and the relationship between emergent fermionic excitations and frustration.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{H.Matsumura_JPSJ_2023,

title = {Large Reduction in the a-axis Knight Shift on UTe2 with Tc = 2.1 K},

author = {H Matsumura and H Fujibayashi and K Kinjo and S Kitagawa and K Ishida and Y Tokunaga and H Sakai and S Kambe and A Nakamura and Y Shimizu and Y Homma and D Li and F Honda and D Aoki },

url = {https://arxiv.org/abs/2305.01200},

doi = {10.7566/JPSJ.92.063701},

year  = {2023},

date = {2023-05-16},

urldate = {2023-05-16},

journal = {Journal of the Physical Society of Japan},

volume = {92},

pages = {063701},

abstract = {Spin susceptibility in the superconducting (SC) state was measured in the higher-quality sample of uranium-based superconductor UTe2 by using Knight-shift measurements for a magnetic field H along all three crystalline axes. In the higher-quality sample, the SC transition temperature Tc is about 2.1 K, and the residual electronic term in the specific heat is almost zero. The NMR linewidth becomes narrower and is almost half of that in the previous sample with Tc ∼ 1.6 K when H || a and c. Although the Knight-shift behavior was not so different from the previous results for H || b, and c, a large reduction in Knight shift along the a axis was observed, in contrast with the previous a-axis Knight shift result. We discuss the origin of the difference between the previous and present results, and the possible SC state derived from the present results.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{H.Fujibayashi_JPSJ_2023,

title = {Low-Temperature Magnetic Fluctuations Investigated by 125Te-NMR on the Uranium-Based Superconductor UTe2},

author = {H Fujibayashi and K Kinjo and G Nakamine and S Kitagawa and K Ishida and Y Tokunaga and H Sakai and S Kambe and A Nakamura and Y Shimizu and Y Homma and D Li and F Honda and D Aoki},

doi = {10.7566/JPSJ.92.053702},

year  = {2023},

date = {2023-04-26},

urldate = {2023-04-26},

journal = {Journal of the Physical Society of Japan},

volume = {92},

pages = {053702},

abstract = {To investigate the static and dynamic magnetic properties on the uranium-based superconductor UTe2, we measured the NMR Knight shift K and the nuclear spin–lattice relaxation rate 1/T1 in H || a by 125Te-NMR on a 125Te-enriched single-crystal sample. 1/T1T in H || a is much smaller than 1/T1T in H || b and c, and magnetic fluctuations along each axis are derived from the 1/T1T measured in H parallel to all three crystalline axes. The magnetic fluctuations are almost identical at two Te sites and isotropic at high temperatures, but become anisotropic below 40 K, where heavy-fermion state is formed. The character of magnetic fluctuations in UTe2 is discussed with the comparison to its static susceptibility and the results on other U-based superconductors. It is considered that the magnetic fluctuations probed with the NMR measurements are determined by the magnetic properties inside the two-leg ladder formed by U atoms, which are dominated by the qa = 0 ferromagnetic fluctuations.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{S.Ogata_PRL_2023,

title = {Parity transition of spin-singlet superconductivity using sub-lattice degrees of freedom},

author = {S Ogata and S Kitagawa and K Kinjo and K Ishida and M Brando and E Hassinger and C Geibel and S Khim},

url = {https://arxiv.org/abs/2304.10032},

doi = {10.1103/PhysRevLett.130.166001},

year  = {2023},

date = {2023-04-19},

urldate = {2023-04-19},

journal = {Physical Review Letters},

volume = {130},

pages = {166001 },

abstract = {Recently, a superconducting (SC) transition from low-field (LF) to high-field (HF) SC states was reported in CeRh2As2, indicating the existence of multiple SC states. It has been theoretically noted that the existence of two Ce sites in the unit cell, the so-called sub-lattice degrees of freedom owing to the local inversion symmetry breaking at the Ce sites, can lead to the appearance of multiple SC phases even under an interaction inducing spin-singlet superconductivity. CeRh2As2 is considered as the first example of multiple SC phases owing to this sub-lattice degree of freedom. However, microscopic information about the SC states has not yet been reported. In this study, we measured the SC spin susceptibility at two crystallographically inequivalent As sites using nuclear magnetic resonance for various magnetic fields. Our experimental results strongly indicate a spin-singlet state in both SC phases. In addition, the antiferromagnetic phase, which appears within the SC phase, only coexists with the LF SC phase; there is no sign of magnetic ordering in the HF SC phase. The present work reveals unique SC properties originating from the locally noncentrosymmetric characteristics. },

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{K.Kinjo_PRB_2023,

title = {Change of superconducting character in UTe2 induced by magnetic field},

author = {K Kinjo and H Fujibayashi and S Kitagawa and K Ishida and Y Tokunaga and H Sakai and S Kambe and A Nakamura and Y Shimizu and Y Homma and D. X Li and F Honda and D Aoki and K Hiraki and M Kimata and T Sasaki},

url = {https://arxiv.org/abs/2206.02444},

doi = {10.1103/PhysRevB.107.L060502},

year  = {2023},

date = {2023-02-10},

urldate = {2023-02-10},

journal = {Physical Review B},

volume = {107},

issue = {06},

pages = {L060502},

abstract = {UTe2 is a recently discovered spin-triplet superconductor. One of the characteristic features of UTe2 is a magnetic field (H)-boosted superconductivity >16 T when H is applied exactly parallel to the b axis. To date, this superconducting (SC) state has not been thoroughly investigated, and the SC properties as well as the spin state of this high-H SC (HHSC) phase are not well understood. In this letter, we performed AC magnetic susceptibility and nuclear magnetic resonance measurements and found that, up to 24.8 T, the HHSC state has bulk nature and is quite sensitive to the H angle and that its SC character is different from that in the low-H SC (LHSC) state. The dominant spin component of the spin-triplet pair is along the a axis in the LHSC state but is changed in the HHSC state along the b axis. Our results indicate that H -induced multiple SC states originate from the remaining spin degrees of freedom.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{CaSb2ARPES,

title = {Fermiology of a topological line-nodal compound  CaSb2  and its implication to superconductivity: Angle-resolved photoemission study},

author = {C-Wen Chuang and S Souma and A Moriya and K Nakayama and A Ikeda and M Kawaguchi and K Obata and S Ranjan Saha and H Takahashi and S Kitagawa and K Ishida and K Tanaka and M Kitamura and K Horiba and H Kumigashira and T Takahashi and S Yonezawa and J Paglione and Y Maeno and T Sato},

url = {https://arxiv.org/abs/2211.15083},

doi = {10.1103/PhysRevMaterials.6.104203},

year  = {2022},

date = {2022-10-24},

urldate = {2022-10-24},

journal = {Physical Review Materials},

volume = {6},

issue = {10},

pages = {104203},

abstract = {We performed angle-resolved photoemission spectroscopy with microfocused beam on a topological line-nodal compound CaSb2 which undergoes a superconducting transition at the onset Tc ∼ 1.8 K, to clarify the Fermi-surface topology relevant to the occurrence of superconductivity. We found that a three-dimensional hole pocket at the Γ point is commonly seen for two types of single-crystalline samples fabricated by different growth conditions. On the other hand, the carrier-doping level estimated from the position of the chemical potential was found to be sensitive to the sample fabrication condition. The cylindrical electron pocket at the Y(C) point predicted by the calculations is absent in one of the two samples, despite the fact that both samples commonly show superconductivity with similar Tc's. This suggests a key role of the three-dimensional hole pocket to the occurrence of superconductivity, and further points to an intriguing possibility to control the topological nature of superconductivity by carrier tuning in CaSb2.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{M.Manago_JPSJ_2022,

title = {Site Split of Antiferromagnetic α-Mn Revealed by 55Mn Nuclear Magnetic Resonance},

author = {M Manago and G Motoyama and S Nishigori and K Fujiwara and K Kinjo and S Kitagawa and K Ishida and K Akiba and S Araki and T C Kobayashi and Hisatomo Harima},

url = {https://arxiv.org/abs/2210.02754},

doi = {10.7566/JPSJ.91.113701},

year  = {2022},

date = {2022-10-06},

urldate = {2022-10-06},

journal = {Journal of the Physical Society of Japan},

volume = {91},

issue = {11},

pages = {113701},

abstract = {The magnetic structure of antiferromagnetic α-Mn has been unclarified for almost 70 years since its magnetism was discovered. We measured the zero-field nuclear magnetic resonance spectra of antiferromagnetic α-Mn to obtain further insight into magnetism below TN = 95 K. The site II spectra split into two sites with five subpeaks owing to quadrupole interaction, and this shows that the ordered moments at site II are slightly tilted from the [001] direction. The site III spectra revealed that this site splits into four sites below TN. These findings clearly demonstrate that the antiferromagnetic α-Mn symmetry is lower than previously considered.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{S.Kitagawa_PRB_2022,

title = {Enhancement of charge-neutral fermionic excitations near the spin-flop transition in the magnetic Kondo material YbIr3Si7},

author = {S Kitagawa and T Kobayashi and F Hori and K Ishida and A H Nevidomskyy and L Qian and E Morosan},

url = {https://doi.org/10.1103/PhysRevB.106.L100405

https://arxiv.org/abs/2209.10844},

doi = {10.1103/PhysRevB.106.L100405},

year  = {2022},

date = {2022-09-16},

urldate = {2022-09-16},

journal = {Physical Review B},

volume = {106},

pages = {L100405},

abstract = {The new Kondo material YbIr3Si7, similar to other Kondo insulators, has been reported to exhibit charge-neutral fermionic excitations through measurements of specific heat and thermal conductivity at low temperatures. 

We performed 29Si−NMR on YbIr3Si7 to investigate the magnetic response of charge-neutral fermions from a microscopic perspective. In low magnetic fields parallel to the c axis, a single NMR peak in the paramagnetic state splits into three peaks below TN. 

In contrast, only a slight shift of the single NMR peak was observed in high magnetic fields. This spectral change as a function of the c

-axis magnetic field is interpreted as a spin-flop transition, at which the magnetic moments oriented along the c axis antiferromagnetic (AFM-I) phase are rotated to the ab plane with a ferromagnetic component along the c-axis (AFM-II phase). In the vicinity of the spin-flop magnetic field 

HM, the nuclear spin-lattice relaxation rate 1/T1 was found to be proportional to temperature at low temperatures, indicating the existence of charge-neutral fermions. Furthermore, a peak of 1/T1 versus the c-axis magnetic field suggests that the charge-neutral fermions in YbIr3Si7 are closely related to its magnetic properties. Our findings shed light on the origin of charge-neutral fermions in insulators.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{T.Taniguchi_JPSJ_2022,

title = {139La-NMR Study of Spin Dynamics Coupled with Hole Mobility in T*-type La0.86Eu0.86Sr0.28CuO4−δ},

author = {T Taniguchi and S Kitagawa and K Ishida and S Asano and K Kudo and M Takahama and P Xie and T Noji and M Fujita},

url = {https://arxiv.org/abs/2110.12333},

doi = {10.7566/JPSJ.91.074710},

year  = {2022},

date = {2022-06-22},

urldate = {2022-06-22},

journal = {Journal of the Physical Society of Japan},

volume = {91},

issue = {07},

pages = {074710},

abstract = {In T*-type cuprate oxides with five-oxygen coordination, the relationship between spin correlations and doped carriers has not been well understood. To clarify this relationship, and the magnetic and superconducting (SC) properties of T*-type cuprate oxides, we performed 139La-nuclear magnetic resonance (NMR) and electrical resistivity measurements on as-sintered (AS) and oxidation-annealed (OA) polycrystalline T*-type La0.86Eu0.86S0.28CuO4 (LESCO) to investigate its magnetic and SC properties. Upon cooling, the NMR spectrum of AS LESCO broadened below 3 K, where the nuclear spin–lattice relaxation rate 1/T1 with respect to the temperature exhibited a maximum, indicating the appearance of static magnetism. The temperature dependence of 1/T1 between 3 and 20 K was similar to that of resistivity, which displays semiconducting behavior. Furthermore, it was found that the energy scales of the transport gap and spin dynamics were comparable. These results suggest a close connection between the mobility of the doped carriers and low-energy spin dynamics, as reported for lightly doped T-type La2−xSrxCuO4. In the OA SC sample, we confirmed the absence of a magnetic order and the linear relation between 1/T1 and T above 10 K. These results suggest that the magnetic state coupled with holes markedly changes to the weakly correlated metallic state by oxidation annealing in the T*-type LESCO with x = 0.28.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{K.Kinjo_Science_2022,

title = {Superconducting spin smecticity evidencing the Fulde-Ferrell-Larkin-Ovchinnikov state in Sr2RuO4},

author = {K Kinjo and M Manago and S Kitagawa and Z Q Mao and S Yonezawa and Y Maeno and K Ishida},

url = {https://www.science.org/doi/10.1126/science.abb0332},

doi = {10.1126/science.abb0332},

year  = {2022},

date = {2022-04-21},

urldate = {2022-04-21},

journal = {Science},

volume = {376},

issue = {6591},

pages = {397-400},

abstract = {Translational symmetry breaking is antagonistic to static fluidity but can be realized in superconductors, which host a quantum-mechanical coherent fluid formed by electron pairs. A peculiar example of such a state is the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, induced by a time-reversal symmetry–breaking magnetic field applied to spin-singlet superconductors. This state is intrinsically accompanied by the superconducting spin smecticity, spin density–modulated fluidity with spontaneous translational-symmetry breaking. Detection of such spin smecticity provides unambiguous evidence for the FFLO state, but its observation has been challenging. Here, we report the characteristic “double-horn” nuclear magnetic resonance spectrum in the layered superconductor Sr2RuO4 near its upper critical field, indicating the spatial sinusoidal modulation of spin density that is consistent with superconducting spin smecticity. Our work reveals that Sr2RuO4 provides a versatile platform for studying FFLO physics.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{K.Kinjo_PRB_2022,

title = {Drastic change in magnetic anisotropy of UTe2 under pressure revealed by 125Te -NMR},

author = {K Kinjo and H Fujibayashi and G Nakamine and S Kitagawa and K Ishida and Y Tokunaga and H Sakai and S Kambe and A Nakamura and Y Shimizu and Y Homma and D Li and F Honda and D Aoki},

url = {https://arxiv.org/abs/2203.12934},

doi = {10.1103/PhysRevB.105.L140502},

year  = {2022},

date = {2022-04-06},

urldate = {2022-04-06},

journal = {Physical Review B},

volume = {105},

issue = {14},

pages = {L140502},

abstract = {To investigate the normal-state magnetic properties of UTe2 under pressure, we perform 125Te nuclear magnetic resonance (NMR) measurements up to 2 GPa. Below 1.2 GPa, the b-axis NMR Knight shift shows a broad maximum at the so-called T_chimax on cooling, which is consistent with the magnetization measurement under pressure. T_chimax decreases with increasing pressure and disappears at the critical pressure Pc = 1.7 GPa, above which superconductivity is destroyed. This tendency is also observed in the temperature dependence of the nuclear spin-lattice relaxation rate 1/T1. At low pressures, 1/T1 shows a conventional Fermi-liquid behavior (1/T1T = constant) at low temperatures, indicating the formation of the heavy-fermion state. Above Pc, 1/T1T follows a 1/T behavior without any crossover to the heavy-fermion state down to the lowest temperature (~3 K). In addition, the NMR signals disappear below 3 K, due to the influence of the magnetically ordered moments. From the pressure dependence of the T_chimax and Knight shift, it was found that the Fermi surface character is abruptly changed at Pc, and that superconductivity is observed only in the heavy-fermion state.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Impurity-Robust Bulk Gapless Excitation in the Yb-Based Zigzag Chain Compound YbCuS2},

author = {F Hori and K Kinjo and S Kitagawa and K Ishida and Y Ohmagari and T Onimaru},

doi = {10.1088/1742-6596/2164/1/012027},

year  = {2022},

date = {2022-03-17},

journal = {J. Phys.: Conf. Ser.},

volume = {2164},

pages = {012027},

abstract = {We have performed 63Cu-nuclear quadrupole resonance (NQR) measurements using a lump sample of the Yb zigzag-chain compound YbCuS2 with a small surface area to investigate the sample dependence of low-temperature magnetic properties in YbCuS2 by comparing with the previous study with different powdered sample. The line width of NQR signals in the present lump sample is larger than that in the previous powdered sample. In addition, the transition temperature TN ∼ 0.92 K in the present lump sample is lower than that in the previous powdered sample (∼ 0.95 K). These results suggest that the quality of the present lump sample is worse than that of the previous powdered sample. However, the T-linear behavior of the nuclear spin-lattice relaxation rate 1/T1 was observed below 0.5 K and the value of 1/T1T in both samples is almost the same even though the sample quality and sample geometry are different. This suggests that T-linear behavior in 1/T1 arises from the impurity-robust bulk gapless excitation inherent in YbCuS2 rather than from sample issues such as the sample quality or geometry.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{H.Fujibayashi_JPSJ_2022,

title = {Superconducting Order Parameter in UTe2 Determined by Knight Shift Measurement},

author = {H Fujibayashi and G Nakamine and K Kinjo and S Kitagawa and K Ishida and Y Tokunaga and H Sakai and S Kambe and A Nakamura and Y Shimizu and Y Homma and D Li and F Honda and D Aoki},

url = {https://doi.org/10.7566/JPSJ.91.043705

https://arxiv.org/abs/2203.08598},

doi = {10.7566/JPSJ.91.043705},

year  = {2022},

date = {2022-03-11},

urldate = {2022-03-11},

journal = {Journal of the Physical Society of Japan},

volume = {91},

number = {04},

pages = {043705},

abstract = {This study investigates the spin susceptibility in U-based superconductor UTe2 in the superconducting (SC) state by using Knight shift measurements for a magnetic field H along the a axis, which is the magnetic easy axis of UTe2. 

Although a tiny anomaly ascribed to the SC diamagnetic effect was observed just below the SC transition temperature Tc, the a-axis Knight shift in the SC state shows no significant decrease, following the extrapolation from the normal-state temperature dependence. 

This indicates that the spin susceptibility is nearly unchanged below Tc. 

Considering the previous Knight shift results for H∥b and H∥c, the dominant SC state is determined to be B3u in the spin-triplet pairing, which is consistent with the spin anisotropy in the normal state. 

The present result shows that UTe2 is a spin-triplet superconductor with spin degrees of freedom.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{S.Kitagawa_JPSJ_2021,

title = {Two-Dimensional XY-Type Magnetic Properties of Locally Noncentrosymmetric Superconductor CeRh2As2},

author = {S Kitagawa and M Kibune and K Kinjo and M Manago and T Taniguchi and K Ishida and M Brando and E Hassinger and C Geibel and S Khim},

url = {https://doi.org/10.7566/JPSJ.91.043702

https://arxiv.org/abs/2203.03184},

doi = {10.7566/JPSJ.91.043702},

year  = {2022},

date = {2022-03-04},

journal = {Journal of the Physical Society of Japan},

volume = {91},

number = {04},

pages = {043702},

abstract = {We performed 75As-NMR measurements to investigate the normal-state magnetic properties of CeRh2As2, a recently-discovered heavy-fermion superconductor. 

The magnitude and temperature dependence of the Knight shift at the As(2) site indicate easy-plane-type magnetic anisotropy in CeRh2As2. 

With regard to spin fluctuations, the temperature dependence of the nuclear spin-lattice relaxation rate 1/T1 arising from the 4f electrons decreases from high-temperature constant behavior on cooling at ∼ 40~K, which is typical behavior of heavy-fermion systems. 

In addition, 1/T1 becomes constant at low temperatures, suggesting spatially two-dimensional antiferromagnetic fluctuations. Two-dimensional magnetic correlations in the real space are quite rare among heavy-fermion superconductors, and they may be a key factor in the unique superconducting multi-phase in CeRh2As2.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{S.Kitagawa_PRL_2022,

title = {Observation of Antiferromagnetic Order as Odd-Parity Multipoles inside the Superconducting Phase in  CeRh2As2},

author = {M Kibune and S Kitagawa and K Kinjo and S Ogata and M Manago and T Taniguchi and K Ishida and M Brando and E Hassinger and H Rosner and C Geibel and S Khim},

url = {https://arxiv.org/abs/2112.07081},

doi = {10.1103/PhysRevLett.128.057002},

year  = {2022},

date = {2022-02-03},

urldate = {2022-02-03},

journal = {Physical Review Letters},

volume = {128},

number = {05},

pages = {057002},

abstract = {Spatial inversion symmetry in crystal structures is closely related to the superconducting (SC) and magnetic properties of materials. Recently, several theoretical proposals that predict various interesting phenomena caused by the breaking of the local inversion symmetry have been presented. However, experimental validation has not yet progressed owing to the lack of model materials. Here we present evidence for antiferromagnetic (AFM) order in CeRh2As2 (SC transition temperature TSC ∼ 0.37 K ), wherein the Ce site breaks the local inversion symmetry. The evidence is based on the observation of different extents of broadening of the nuclear quadrupole resonance spectrum at two crystallographically inequivalent As sites. This AFM ordering breaks the inversion symmetry of this system, resulting in the activation of an odd-parity magnetic multipole. Moreover, the onset of antiferromagnetism TN within an SC phase, with TN < TSC , is quite unusual in systems wherein superconductivity coexists or competes with magnetism. Our observations show that CeRh2As2 is a promising system to study how the absence of local inversion symmetry induces or influences unconventional magnetic and SC states, as well as their interaction.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Y.Tokunaga_JPSJ_2022,

title = {Slow Electronic Dynamics in the Paramagnetic State of UTe2},

author = {Y Tokunaga and H Sakai and S Kambe and Y Haga and Y Tokiwa and P Opletal and H Fujibayashi and K Kinjo and S Kitagawa and K Ishida and A Nakamura and Y Shimizu and Y Homma and D Li and F Honda and D Aoki},

url = {https://arxiv.org/abs/2201.07455},

doi = {10.7566/JPSJ.91.023707},

year  = {2022},

date = {2022-01-27},

urldate = {2022-01-27},

journal = {Journal of the Physical Society of Japan},

volume = {91},

issue = {02},

pages = {023707},

abstract = {125Te NMR experiments in field (H) applied along the easy magnetization axis (the a-axis) revealed slow electronic dynamics developing in the paramagnetic state of UTe2. The observed slow fluctuations are concerned with a successive growth of long-range electronic correlations below 30–40 K, where the spin susceptibility along the hard magnetization axis (the b-axis) shows a broad maximum. The experiments also imply that tiny amounts of disorder or defects locally disturb the long-range electronic correlations and develop an inhomogeneous electronic state at low temperatures, leading to a low temperature upturn observed in the bulk-susceptibility in H || a. We suggest that UTe2 would be located on the paramagnetic side near an electronic phase boundary, where either magnetic or Fermi-surface instability would be the origin of the characteristic fluctuations.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Y.Sato_NComm_2022,

title = {Charge neutral fermions and magnetic field driven instability in insulating YbIr3Si7},

author = {Y Sato and S Suetsugu and T Tominaga and Y Kasahara and S Kasahara and T Kobayashi and S Kitagawa and K Ishida and R Peters and T Shibauchi and A H Nevidomskyy and L Qian and J M Moya and E Morosan and Y Matsuda},

url = {https://arxiv.org/abs/2103.13718},

doi = {10.1038/s41467-021-27541-9},

year  = {2022},

date = {2022-01-19},

urldate = {2022-01-19},

journal = {Nature Communications},

volume = {13},

pages = {394},

abstract = {Kondo lattice materials, where localized magnetic moments couple to itinerant electrons, provide a very rich backdrop for strong electron correlations. They are known to realize many exotic phenomena, with a dramatic example being recent observations of quantum oscillations and metallic thermal conduction in insulators, implying the emergence of enigmatic charge-neutral fermions. Here, we show that thermal conductivity and specific heat measurements in insulating YbIr3Si7 reveal emergent neutral excitations, whose properties are sensitively changed by a field-driven transition between two antiferromagnetic phases. In the low-field phase, a significant violation of the Wiedemann-Franz law demonstrates that YbIr3Si7 is a charge insulator but a thermal metal. In the high-field phase, thermal conductivity exhibits a sharp drop below 300 mK, indicating a transition from a thermal metal into an insulator/semimetal driven by the magnetic transition. These results suggest that spin degrees of freedom directly couple to the neutral fermions, whose emergent Fermi surface undergoes a field-driven instability at low temperatures.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{K.Ishida_PRB_2021,

title = {Pairing interaction in superconducting UCoGe tunable by magnetic field},

author = {K Ishida and S Matsuzaki and M Manago and T Hattori and S Kitagawa and M Hirata and T Sasaki and D Aoki},

url = {https://arxiv.org/abs/2109.14426},

doi = {10.1103/PhysRevB.104.144505},

year  = {2021},

date = {2021-10-18},

journal = {Physical Review B},

volume = {104},

number = {14},

pages = {144505},

abstract = {The mechanism of unconventional superconductivity, such as high-temperature-cuprate, Fe-based, and heavy-fermion superconductors, has been studied as a central issue in condensed-matter physics. Spin fluctuations, instead of phonons, are considered to be responsible for the formation of Cooper pairs, and many efforts have been made to confirm this mechanism experimentally. Although a qualitative consensus seems to have been obtained, experimental confirmation has not yet been achieved. This is owing to a lack of the quantitative comparison between theory and experiments. Here, we show a semiquantitative comparison between the superconducting-transition temperature ( T SC ) and spin fluctuations derived from the NMR experiment on the ferromagnetic (FM) superconductor UCoGe in which the FM fluctuations and superconductivity are tunable by external fields. The enhancement and abrupt suppression of T SC by applied fields, as well as the pressure variation of T SC around the FM criticality are well understood by the change in the FM fluctuations on the basis of the single-band spin-triplet theoretical formalism. The present comparisons strongly support the theoretical formalism of spin-fluctuation-mediated superconductivity, particularly in UCoGe.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{S.Kitagawa_PRB_2021,

title = {Peak in the superconducting transition temperature of the nonmagnetic topological line-nodal material CaSb_{2} under pressure},

author = {S Kitagawa and K Ishida and A Ikeda and M Kawaguchi and S Yonezawa and Y Maeno},

url = {https://arxiv.org/abs/2108.08992},

doi = {10.1103/PhysRevB.104.L060504},

year  = {2021},

date = {2021-08-17},

journal = {Physical Review B},

volume = {104},

number = {6},

pages = {L060504},

abstract = {Investigating the pressure dependence of the superconducting (SC) transition temperature Tc is crucial for understanding the SC mechanism. 

Herein, we report on the pressure dependence of Tc in the nonmagnetic topological line-nodal material CaSb2 based on measurements of electric resistance and alternating current magnetic susceptibility. 

Tc initially increases with increasing pressure and peaks at ~ 3.1 GPa. With a further increase in pressure, Tc decreases and finally becomes undetectable at 5.9 GPa. 

Because no signs of phase transition or Lifshitz transition are observed in the normal state, the peculiar peak structure of Tc suggests that CaSb2 has an unconventional SC character.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{H.Takahashi_JPSJ_2021,

title = {S-Wave Superconductivity in the Dirac Line-Nodal Material CaSb2},

author = {H Takahashi and S Kitagawa and K Ishida and M Kawaguchi and A Ikeda and S Yonezawa and Y Maeno},

url = {https://arxiv.org/abs/2105.13614},

doi = {10.7566/JPSJ.90.073702},

year  = {2021},

date = {2021-06-11},

urldate = {2021-06-11},

journal = {Journal of the Physical Society of Japan},

volume = {90},

pages = {073702},

abstract = {We performed 121/123Sb-nuclear quadrupole resonance (NQR) measurements on the superconducting (SC) line-nodal material CaSb2 in order to investigate electronic properties in the normal and SC states from a microscopic point of view. In the normal state, the nuclear spin–lattice relaxation rate 1/T1 for the Sb(1) site, which is responsible for the line-nodal parts, is approximately proportional to temperature, indicating the conventional Fermi liquid state. From comparison with band structure calculations, it is considered that the NQR properties related to the line-nodal character are hidden because the conventional behavior originating from Fermi-surface parts away from the nodes is dominant. In the SC state, a clear coherence peak just below the transition temperature and an exponential decrease at lower temperatures were observed in 1/T1. These results strongly suggest that conventional s-wave superconductivity with a full gap is realized in CaSb2.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{G.Nakamine_JPSJ_2021,

title = {Inhomogeneous Superconducting State Probed by 125Te NMR on UTe2},

author = {G Nakamine and K Kinjo and S Kitagawa and K Ishida and Y Tokunaga and H Sakai and S Kambe and A Nakamura and Y Shimizu and Y Homma and D Li and F Honda and D Aoki},

url = {https://arxiv.org/abs/2105.11823},

doi = {10.7566/JPSJ.90.064709},

year  = {2021},

date = {2021-05-25},

journal = {Journal of the Physical Society of Japan},

volume = {90},

number = {064709},

pages = {7},

abstract = {UTe2 is a recently discovered promising candidate for a spin-triplet superconductor. In contrast to conventional spin-singlet superconductivity, spin-triplet superconductivity possesses spin and angular momentum degrees of freedom. To detect these degrees of freedom and obtain the solid evidence of spin-triplet superconductivity in UTe2, we have performed 125Te-NMR measurements, which are sensitive to the local spin susceptibility at a nuclear site. We previously reported that the shoulder signal appears in NMR spectra below the superconducting (SC) transition temperature Tc in H || b, and a slight decrease in the Knight shift along the b and c axes (Kb and Kc, respectively) below Tc at a low magnetic field H. Although the decrease in Kc vanished above 5.5 T, the decrease in Kb was independent of H up to 6.5 T. To clarify the origin of the shoulder signal and the trace of the decrease in Kb, we compared the 125Te-NMR spectra obtained when H || b and H || c and measured the 125Te-NMR spectra for H || b up to 14.5 T. The intensity of the shoulder signal observed for H || b has a maximum at ∼6 T and vanishes above 10 T, although the superconductivity is confirmed by the χAC measurements, which can survive up to 14.5 T (maximum H in the present measurement). Moreover, the decrease in Kb in the SC state starts to be small around 7 T and almost zero at 12.5 T. This indicates that the SC spin state gradually changes with the application of H. Meanwhile, in H || c, an unexpected broadening without the shoulder signals was observed below Tc at 1 T, and this broadening was quickly suppressed with increasing H. We construct the H–T phase diagram for H || b and H || c based on the NMR measurements and discuss possible SC states with the theoretical consideration. We suggest that the inhomogeneous SC state characterized by the broadening of the NMR spectrum originates from the spin degrees of freedom.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{G.Nakamine_PRB_2021,

title = {Anisotropic response of spin susceptibility in the superconducting state of  UTe2  probed with  125Te − NMR  measurement},

author = {G Nakamine and K Kinjo and S Kitagawa and K Ishida and Y Tokunaga and H Sakai and S Kambe and A Nakamura and Y Shimizu and Y Homma and D Li and F Honda and D Aoki},

url = {https://arxiv.org/abs/2103.02876},

doi = {10.1103/PhysRevB.103.L100503},

year  = {2021},

date = {2021-03-17},

journal = {Physical Review B},

volume = {103},

number = {10},

pages = {L100503},

abstract = {To investigate spin susceptibility in a superconducting (SC) state, we measured the 125 Te -NMR Knight shifts at magnetic fields ( H ) up to 6.5 T along the b and c axes of single-crystal UTe 2 , a promising candidate for a spin-triplet superconductor. In the SC state, the Knight shifts along the b and c axes ( K b and K c , respectively) decreased slightly, and the decrease in K b was almost constant up to 6.5 T. The reduction in K c decreased with increasing H , and K c was unchanged through the SC transition temperature at 5.5 T, excluding the possibility of spin-singlet pairing. Our results indicate that spin susceptibilities along the b and c axes slightly decrease in the SC state in low H , and the H response of SC spin susceptibility is anisotropic on the b c plane. We discuss the possible d -vector state within the spin-triplet scenario and suggest that the dominant d -vector component for the case of H ∥ b changes above 13 T, where T c increases with increasing H .},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Yamamoto_NC_2020,

title = {Strain-induced creation and switching of anion vacancy layers in perovskite oxynitrides},

author = {T Yamamoto and A Chikamatsu and S Kitagawa and N Izumo and S Yamashita and H Takatsu and M Ochi and T Maruyama and M Namba and W Sun and T Nakashima and F Takeiri and K Fujii and M Yashima and Y Sugisawa and M Sano and Y Hirose and D Sekiba and C M Brown and T Honda and K Ikeda and T Otomo and K Kuroki and K Ishida and T Mori and K Kimoto and T Hasegawa and H Kageyama},

doi = {10.1038/s41467-020-19217-7},

year  = {2020},

date = {2020-11-23},

journal = {Nature Communications},

volume = {11},

pages = {5923},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Okuno_JPSJ_2020,

title = {Magnetic-Field Dependence of Novel Gap Behavior Related to the Quantum-Size Effect},

author = {T Okuno and Y Kinoshita and S Matsuzaki and S Kitagawa and K Ishida and M Hirata and T Sasaki and K Kusada and H Kitagawa},

url = {https://arxiv.org/abs/2007.07688},

doi = {10.7566/JPSJ.89.095002},

year  = {2020},

date = {2020-08-03},

journal = {Journal of the Physical Society of Japan},

volume = {89},

number = {9},

pages = {095002},

abstract = {195Pt-NMR measurements of Pt nanoparticles with a mean diameter of 4.0 nm were performed in a high magnetic field of approximately μ0H = 23.3 T to investigate the low-temperature electronic state of the nanoparticles. The characteristic temperature T*, below which the nuclear spin-lattice relaxation rate 1/T1 deviates from the relaxation rate of the bulk, shows a magnetic-field dependence. This dependence supports the theoretical prediction of the appearance of discrete energy levels.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{S_Kitagawa2020,

title = {Variation in Superconducting Symmetry Against Pressure on Noncentrosymmetric Superconductor Cd_{2}Re_{2}O_{7} Revealed by ^{185/187}Re Nuclear Quadrupole Resonance},

author = {S Kitagawa and K Ishida and T C Kobayashi and Y Matsubayashi and D Hirai and Z Hiroi},

url = {https://arxiv.org/abs/2004.00919},

doi = {10.7566/JPSJ.89.053701},

year  = {2020},

date = {2020-04-02},

journal = {Journal of the Physical Society of Japan},

volume = {89},

number = {5},

pages = {053701},

abstract = {We performed 185/187Re nuclear quadrupole resonance (NQR) measurements under pressure to investigate the superconducting properties of noncentrosymmetric superconductor Cd2Re2O7 under various crystal structures. The pressure dependence of superconducting transition temperature Tc determined through ac susceptibility measurements is consistent with the results of previous resistivity measurements [Kobayashi et al., J. Phys. Soc. Jpn. 80, 023715 (2011)]. Below 2.2 GPa, in the nuclear spin–lattice relaxation rate 1/T1, a clear coherence peak was observed just below Tc, indicating conventional s-wave superconductivity. In contrast, the coherence peak disappears at 3.1 GPa, suggesting a change in superconducting symmetry to the p-wave dominant state against pressure.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Okuno2020,

title = {NMR-based gap behavior related to the quantum size effect},

author = {T Okuno and M Manago and S Kitagawa and K Ishida and K Kusada and H Kitagawa},

url = {https://arxiv.org/abs/2003.07585},

doi = {10.1103/PhysRevB.101.121406},

year  = {2020},

date = {2020-03-30},

journal = {Physical Review B},

volume = {101},

number = {12},

abstract = {We conducted Pt195-NMR measurements on various-diameter Pt nanoparticles coated with polyvinylpyrrolidone in order to detect the quantum size effect and the discrete energy levels in the electron density of states, both of which were predicted by Kubo more than 50 years ago. We succeeded in separating the signals arising from the surface and interior regions and found that the nuclear spin-lattice relaxation rates in both regions show the metallic behavior at high temperatures. Surprisingly, the magnetic fluctuations in both regions exhibited anomalous behavior below the same temperature T∗, which points to a clear size dependence and is well scaled with δKubo. These results suggest that a size-tunable metal-insulator transition occurs in the Pt nanoparticles as a result of the quantum size effect. © 2020 American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Manago2019c,

title = {Superconductivity at the pressure-induced ferromagnetic critical region in UCoGe},

author = {M Manago and S Kitagawa and K Ishida and K Deguchi and N K Sato and T Yamamura},

doi = {10.7566/JPSJ.88.113704},

year  = {2019},

date = {2019-10-23},

journal = {Journal of the Physical Society of Japan},

volume = {88},

number = {11},

abstract = {The phase separation of the ferromagnetic (FM) and paramagnetic (PM) phases in the superconducting (SC) state of UCoGe at the FM critical region was investigated using 59Co nuclear quadrupole resonance (NQR) technique by taking advantage of its site-selective feature. The NQR measurements revealed that the first-order quantum phase transition occurs between the FM and the PM states. The nuclear spin–lattice relaxation rate 1=T1 exhibited a clear drop at the SC state in the PM phase, whereas it was not detected in the FM phase, which indicates that the superconductivity in the FM phase becomes weaker at the FM critical region due to the presence of the PM SC state. This result suggests that the SC condensation energy of the PM SC state is equal or larger than that of the FM SC state in this region. The pressure–temperature phase diagram of UCoGe was modified by taking the results from this study into account. ©2019 The Physical Society of Japan},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Nakamine2019b,

title = {Superconducting Properties of Heavy Fermion UTe_{2} Revealed by ^{125}Te-nuclear Magnetic Resonance},

author = {G Nakamine and S Kitagawa and K Ishida and Y Tokunaga and H Sakai and S Kambe and A Nakamura and Y Shimizu and Y Homma and D Li and F Honda and D Aoki},

doi = {10.7566/JPSJ.88.113703},

year  = {2019},

date = {2019-10-17},

journal = {Journal of the Physical Society of Japan},

volume = {88},

number = {11},

abstract = {We have performed the 125Te-nuclear magnetic resonance (NMR) measurement in the field along the b axis on the newly discovered superconductor UTe2, which is a candidate of a spin-triplet superconductor. The nuclear spin–lattice relaxation rate divided by temperature 1=T1T abruptly decreases below a superconducting (SC) transition temperature Tc without showing a coherence peak, indicative of UTe2 being an unconventional superconductor. It was found that the temperature dependence of 1=T1T in the SC state cannot be understood by a single SC gap behavior but can be explained by a two SC gap model. The Knight shift, proportional to the spin susceptibility, decreases below Tc, but the magnitude of the decrease is much smaller than the decrease expected in the spin-singlet pairing. Rather, the small Knight-shift decrease as well as the absence of the Pauli-depairing effect can be interpreted by the spin triplet scenario. © 2019 Society The Author(s) of Japan.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kitagawa2019,

title = {Universal relationship between low-energy antiferromagnetic fluctuations and superconductivity in BaFe_{2}(As_{1-x}P_{x})_{2}},

author = {S Kitagawa and T Kawamura and K Ishida and Y Mizukami and S Kasahara and T Shibauchi and T Terashima and Y Matsuda},

doi = {10.1103/PhysRevB.100.060503},

year  = {2019},

date = {2019-08-12},

journal = {Physical Review B},

volume = {100},

number = {6},

abstract = {To identify the key parameter for optimal superconductivity in iron pnictides, we measured the P31-NMR relaxation rate on BaFe2(As1-xPx)2(x=0.22 and 0.28) under pressure and compared the effects of chemical substitution and physical pressure. For x=0.22, structural and antiferromagnetic (AFM) transition temperatures both show minimal changes with pressure up to 2.4 GPa, whereas the superconducting transition temperature Tc increases to twice its former value. In contrast, for x=0.28 near the AFM quantum critical point (QCP), the structural phase transition is quickly suppressed by pressure and Tc reaches a maximum. The analysis of the temperature-dependent nuclear relaxation rate indicates that these contrasting behaviors can be quantitatively explained by a single curve of the Tc dome as a function of Weiss temperature θ, which measures the distance to the QCP. Moreover, the Tc-θ curve under pressure precisely coincides with that with a chemical substitution, which is indicative of the existence of a universal relationship between low-energy AFM fluctuations and superconductivity on BaFe2(As1-xPx)2. © 2019 American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Asano2019,

title = {Oxidation annealing effects on the spin-glass-like magnetism and appearance of superconductivity in T*-type La_{1−x/2}Eu_{1−x/2}Sr_{x}CuO_{4} (0.14 ≤ x ≤ 0.28)},

author = {S Asano and K M Suzuki and K Kudo and I Watanabe and A Koda and R Kadono and T Noji and Y Koike and T Taniguchi and S Kitagawa and K Ishida and M Fujita},

doi = {10.7566/JPSJ.88.084709},

year  = {2019},

date = {2019-07-23},

journal = {Journal of the Physical Society of Japan},

volume = {88},

number = {8},

abstract = {We investigated the magnetism and superconductivity in as-sintered (AS) and oxidation annealed (OA) T+-type La1−x=2Eu1−x=2SrxCuO4 (LESCO) with 0.14 ≤ x ≤ 0.28 by the first comprehensive muon spin rotation=relaxation (μSR), magnetic susceptibility, and electrical resistivity measurements. In OA superconducting samples, no evidence of magnetic order was observed, whereas AS semiconducting samples exhibited evidence of a disordered magnetic state in the measured temperature range between ∼4 and ∼8 K. Therefore, the ground state in LESCO drastically varies with oxidation annealing and the magnetic phase competitively exists with the superconducting (SC) phase. The magnetic phase in the AS LESCO is quite robust against Sr doping, while the SC phase degrades with increasing x. A monotonous decrease of the SC transition temperature from 24.5 K in x = 0.14 to 9.0 K in x = 0.28 suggests the disappearance of the SC phase at x ∼ 0.34. Furthermore, we clarified the simultaneous development of (quasi) static magnetism and the electrical resistivity at a low temperature in AS samples, suggesting the inducement of magnetism by the suppression of carrier mobility. The variation in magnetism due to annealing is discussed from a viewpoint of structural defects, which was previously reported from neutron diffraction measurements. ©2019 The Physical Society of Japan},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Manago2019,

title = {Spin-triplet superconductivity in the paramagnetic UCoGe under pressure studied by ^{59}Co NMR},

author = {M Manago and S Kitagawa and K Ishida and K Deguchi and N K Sato and T Yamamura},

doi = {10.1103/PhysRevB.100.035203},

year  = {2019},

date = {2019-07-12},

journal = {Physical Review B},

volume = {100},

number = {3},

abstract = {A Co59 nuclear magnetic resonance (NMR) measurement was performed on the single-crystalline ferromagnetic (FM) superconductor UCoGe under a pressure of 1.09 GPa, where the FM state is suppressed and superconductivity occurs in the paramagnetic (PM) state, to study the superconducting (SC) state in the PM state. Co59-NMR spectra became broader but hardly shifted across the SC transition temperature. The Knight-shift change determined from fitting the spectral peak with a Gaussian was much smaller than the spin part of the Knight shift; this is in good agreement with the spin-triplet pairing suggested from the large upper critical field. The spectrum broadening in the SC state cannot be attributed to the SC diamagnetic effect but is related to the properties of spin-triplet superconductivity. The origins of the broadening are discussed herein. © 2019 American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Tokunaga2019,

title = {^{125}Te-NMR study on a single crystal of heavy fermion superconductor UTe_{2}},

author = {Y Tokunaga and H Sakai and S Kambe and T Hattori and N Higa and G Nakamine and S Kitagawa and K Ishida and A Nakamura and Y Shimizu and Y Homma and D Li and F Honda and D Aoki},

doi = {10.7566/JPSJ.88.073701},

year  = {2019},

date = {2019-06-24},

journal = {Journal of the Physical Society of Japan},

volume = {88},

number = {7},

abstract = {We report 125Te NMR studies on a newly discovered heavy fermion superconductor UTe2. Using a single crystal, we have measured the 125Te NMR Knight shift K and spin-lattice relaxation rate 1=T1 for fields along the three orthorhombic crystal axes. The data confirm a moderate Ising anisotropy for both the static (K) and dynamical susceptibilities (1=T1) in the paramagnetic state above about 20 K. Around 20 K, however, we have observed a sudden loss of NMR spin-echo signal due to sudden enhancement of the NMR spin-spin relaxation rate 1=T2, when the field is applied along the easy axis of magnetization (= a-axis). This behavior suggests the development of longitudinal magnetic fluctuations along the a-axis at very low frequencies below 20 K. © 2019 The Physical Society of Japan},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kinjo2019,

title = {Magnetic field effect on s-wave superconductor LaRu_{4}P_{12} Studied by ^{31}P-NMR},

author = {K Kinjo and S Kitagawa and Y Nakai and K Ishida and H Sugawara and H Sato},

doi = {10.7566/JPSJ.88.065002},

year  = {2019},

date = {2019-05-30},

journal = {Journal of the Physical Society of Japan},

volume = {88},

number = {6},

abstract = {We have performed 31P-NMR measurements on the s-wave superconductor LaRu4P12 to investigate the magnetic field effect of the nuclear spin–lattice relaxation rate 1=T1 on a conventional full-gap superconductor. With increasing magnetic field, the Hebel–Slichter peak immediately below Tc in 1=T1 was suppressed, and the magnetic field dependence of 1=T1 at 0.8K, well below Tc, was proportional to H2. These behaviors can be fully understood by the orbital pair-breaking effect in a single-band s-wave superconductor. © 2019 The Physical Society of Japan},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Nakamine2019,

title = {Modification of magnetic fluctuations by interfacial interactions in artificially engineered heavy-fermion superlattices},

author = {G Nakamine and T Yamanaka and S Kitagawa and M Naritsuka and T Ishii and T Shibauchi and T Terashima and Y Kasahara and Y Matsuda and K Ishida},

doi = {10.1103/PhysRevB.99.081115},

year  = {2019},

date = {2019-02-19},

journal = {Physical Review B},

volume = {99},

number = {8},

abstract = {Recent progress in the fabrication techniques of superlattices (SLs) has made it possible to sandwich several-layer-thick block layers (BLs) of heavy-fermion superconductor CeCoIn5 between conventional-metal YbCoIn5 BLs or spin-density-wave-metal CeRhIn5 BLs of a similar thickness. However, the magnetic state in each BL, particularly at the interface, is not yet understood, as experimental techniques applicable to the SL system are limited. Here, we report measurements of Co59 nuclear magnetic resonance, which is a microscopic probe of the magnetic properties inside the target BLs. In the CeCoIn5/YbCoIn5 SL, the low-temperature magnetic fluctuations of the CeCoIn5 BL are weakened as expected from the Rashba spin-orbit effect. However, in the CeCoIn5/CeRhIn5 SL, the fluctuations show an anomalous enhancement below 6 K, highlighting the importance of the magnetic proximity effect occurring near a magnetic-ordering temperature TN∼3 K of the CeRhIn5 BL. We suggest that the magnetic properties of the BLs can be altered by the interfacial interaction, which is an alternative route to modify the magnetic properties. © 2019 American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Manago2019b,

title = {Enhancement of superconductivity by pressure-induced critical ferromagnetic fluctuations in UCoGe},

author = {M Manago and S Kitagawa and K Ishida and K Deguchi and N K Sato and T Yamamura},

doi = {10.1103/PhysRevB.99.020506},

year  = {2019},

date = {2019-01-24},

journal = {Physical Review B},

volume = {99},

number = {2},

abstract = {A Co59 nuclear quadrupole resonance (NQR) was performed on a single-crystalline ferromagnetic (FM) superconductor UCoGe under pressure. The FM phase vanished at a critical pressure Pc, and the NQR spectrum just below Pc showed phase separation of the FM and paramagnetic (PM) phases below Curie temperature TCurie, suggesting first-order FM quantum phase transition (QPT). We found that the internal field was absent above Pc, but the superconductivity is almost unchanged. This result suggests the existence of the nonunitary to unitary transition of the superconductivity around Pc. Nuclear spin-lattice relaxation rate 1/T1 showed the FM critical fluctuations around Pc, which persist above Pc and are clearly related to superconductivity in the PM phase. This FM QPT is understood to be a weak first order with critical fluctuations. 1/T1 sharply decreased in the superconducting (SC) state above Pc with a single component, in contrast to the two-component 1/T1 in the FM SC state, indicating that the inhomogeneous SC state is a characteristic feature of the FM SC state in UCoGe. © 2019 American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kitagawa2018,

title = {Normal-state properties of the antiperovskite oxide Sr3-xSnO revealed by Sn 119 -NMR},

author = {S Kitagawa and K Ishida and M Oudah and J N Hausmann and A Ikeda and S Yonezawa and Y Maeno},

url = {http://hdl.handle.net/2433/235675

https://arxiv.org/abs/1809.09275},

doi = {10.1103/PhysRevB.98.100503},

year  = {2018},

date = {2018-09-21},

journal = {Physical Review B},

volume = {98},

number = {10},

pages = {100503(R)},

abstract = {We have performed Sn119-NMR measurements on the antiperovskite oxide superconductor Sr3-xSnO to investigate how its normal state changes with the Sr deficiency. A two-peak structure was observed in the NMR spectra of all the measured samples. This suggests that the phase separation tends to occur between the nearly stoichiometric and heavily Sr-deficient Sr3-xSnO phases. The measurement of the nuclear spin-lattice relaxation rate 1/T1 indicates that the Sr-deficient phase shows a conventional metallic behavior due to the heavy hole doping. In contrast, the nearly stoichiometric phase exhibits unusual temperature dependence of 1/T1, attributable to the presence of a Dirac-electron band. © 2018 American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kitagawa2018b,

title = {Nematic transition and highly two-dimensional superconductivity in BaTi2Bi2 O revealed by Bi 209 -nuclear magnetic resonance/nuclear quadrupole resonance measurements},

author = {S Kitagawa and K Ishida and W Ishii and T Yajima and Z Hiroi},

doi = {10.1103/PhysRevB.98.220507},

year  = {2018},

date = {2018-01-01},

journal = {Physical Review B},

volume = {98},

number = {22},

abstract = {In this Rapid Communication, a set of Bi209-nuclear magnetic resonance (NMR)/nuclear quadrupole resonance (NQR) measurements has been performed to investigate the physical properties of superconducting (SC) BaTi2Bi2O from a microscopic point of view. The NMR and NQR spectra at 5 K can be reproduced with a nonzero in-plane anisotropic parameter η, indicating the breaking of the in-plane fourfold symmetry at the Bi site without any magnetic order, i.e., "the electronic nematic state." In the SC state, the nuclear spin-lattice relaxation rate divided by temperature, 1/T1T, does not change even below Tc, while a clear SC transition was observed with a diamagnetic signal. This observation can be attributed to the strong two dimensionality in BaTi2Bi2O. Comparing the NMR/NQR results among BaTi2Pn2O (Pn=As, Sb, and Bi), it was found that the normal and SC properties of BaTi2Bi2O were considerably different from those of BaTi2Sb2O and BaTi2As2O, which might explain the two-dome structure of Tc in this system. © 2018 American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Smidman20182930,

title = {Interplay between unconventional superconductivity and heavy-fermion quantum criticality: CeCu2Si2 versus YbRh2Si2},

author = {M Smidman and O Stockert and J Arndt and G M Pang and L Jiao and H Q Yuan and H A Vieyra and S Kitagawa and K Ishida and K Fujiwara and T C Kobayashi and E Schuberth and M Tippmann and L Steinke and S Lausberg and A Steppke and M Brando and H Pfau and U Stockert and P Sun and S Friedemann and S Wirth and C Krellner and S Kirchner and E M Nica and R Yu and Q Si and F Steglich},

doi = {10.1080/14786435.2018.1511070},

year  = {2018},

date = {2018-01-01},

journal = {Philosophical Magazine},

volume = {98},

number = {32},

pages = {2930-2963},

abstract = {In this paper the low-temperature properties of two isostructural canonical heavy-fermion compounds are contrasted with regards to the interplay between antiferromagnetic (AF) quantum criticality and superconductivity. For CeCu2Si2, fully-gapped d-wave superconductivity forms in the vicinity of an itinerant three-dimensional heavy-fermion spin-density-wave (SDW) quantum critical point (QCP). Inelastic neutron scattering results highlight that both quantum critical SDW fluctuations as well as Mott-type fluctuations of local magnetic moments contribute to the formation of Cooper pairs in CeCu2Si2. In YbRh2Si2, superconductivity appears to be suppressed at T ⪆ 10 mK by AF order (TN = 70 mK). Ultra-low temperature measurements reveal a hybrid order between nuclear and 4f-electronic spins, which is dominated by the Yb-derived nuclear spins, to develop at TA slightly above 2 mK. The hybrid order turns out to strongly compete with the primary 4f-electronic order and to push the material towards its QCP. Apparently, this paves the way for heavy-fermion superconductivity to form at Tc = 2 mK. Like the pressure – induced QCP in CeRhIn5, the magnetic field – induced one in YbRh2Si2 is of the local Kondo-destroying variety which corresponds to a Mott-type transition at zero temperature. Therefore, these materials form the link between the large family of about fifty low-T unconventional heavy – fermion superconductors and other families of unconventional superconductors with higher Tcs, notably the doped Mott insulators of the cuprates, organic charge-transfer salts and some of the Fe-based superconductors. Our study suggests that heavy-fermion superconductivity near an AF QCP is a robust phenomenon. © 2018, © 2018 Informa UK Limited, trading as Taylor & Francis Group.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kitagawa2018c,

title = {Evidence for the Presence of the Fulde-Ferrell-Larkin-Ovchinnikov State in CeCu2Si2 Revealed Using Cu 63 NMR},

author = {S Kitagawa and G Nakamine and K Ishida and H S Jeevan and C Geibel and F Steglich},

doi = {10.1103/PhysRevLett.121.157004},

year  = {2018},

date = {2018-01-01},

journal = {Physical Review Letters},

volume = {121},

number = {15},

abstract = {Nuclear magnetic resonance measurements were performed on CeCu2Si2 in the presence of a magnetic field close to the upper critical field μ0Hc2 in order to investigate its superconducting (SC) properties near pair-breaking fields. In lower fields, the Knight shift and nuclear spin-lattice relaxation rate divided by temperature 1/T1T abruptly decreased below the SC transition temperature Tc(H), a phenomenon understood within the framework of conventional spin-singlet superconductivity. In contrast, 1/T1T was enhanced just below Tc(H) and exhibited a broad maximum when magnetic fields close to μ0Hc2(0) were applied parallel or perpendicular to the c axis; although the Knight shift decreased just below Tc(H). This enhancement of 1/T1T, which was recently observed in the organic superconductor κ-(BEDT-TTF)2Cu(NCS)2, suggests the presence of high-density Andreev bound states in the inhomogeneous SC region, a hallmark of the Fulde-Ferrell-Larkin-Ovchinnikov phase. © 2018 American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Yamamoto20181566,

title = {Selective Hydride Occupation in BaVO3-xHx (0.3 ≤ × ≤ 0.8) with Faceand Corner-Shared Octahedra},

author = {T Yamamoto and K Shitara and S Kitagawa and A Kuwabara and M Kuroe and K Ishida and M Ochi and K Kuroki and K Fujii and M Yashima and C M Brown and H Takatsu and C Tassel and H Kageyama},

doi = {10.1021/acs.chemmater.7b04571},

year  = {2018},

date = {2018-01-01},

journal = {Chemistry of Materials},

volume = {30},

number = {5},

pages = {1566-1574},

abstract = {A growing number of transition metal oxyhydrides have recently been reported, but they are all confined to perovskite-related structures with corner-shared octahedra. Using high pressure synthesis, we have obtained vanadium oxyhydrides BaVO3-xHx (0.3 ≤ x ≤ 0.8) with a 6H-type hexagonal layer structure consisting of face-shared as well as corner-shared octahedra. Synchrotron X-ray and neutron diffraction measurements revealed that, in BaVO2.7H0.3, H- anions are located selectively at the face-shared sites, as supported by DFT calculations, while BaVO2.2H0.8 contains H- anions at both sites though the face-shared preference is partially retained. The selective hydride occupation for BaVO2.7H0.3 appears to suppress electron hopping along the c axis, making this material a quasi-two-dimensional metal characterized by anomalous temperature dependence of the electrical resistivity and strong antiferromagnetic fluctuations. In contrast, the anion disordered BaVO3-xHx in hexagonal (x ≈ 0.8) and cubic (x ≈ 0.9) forms exhibits a semiconducting behavior. This study offers a wide opportunity to develop transition metal oxyhydrides having various polyhedral linkages, along with site preference of H/O anions, aimed at finding interesting phenomena. © 2018 American Chemical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kitagawa2018d,

title = {Spatially inhomogeneous superconducting state near Hc2 in UPd2Al3},

author = {S Kitagawa and R Takaki and M Manago and K Ishida and N K Sato},

doi = {10.7566/JPSJ.87.013701},

year  = {2018},

date = {2018-01-01},

journal = {Journal of the Physical Society of Japan},

volume = {87},

number = {1},

abstract = {We have performed 27Al-NMR measurements on single-crystalline UPd2Al3 with the field parallel to the c-axis to investigate the superconducting (SC) properties near the upper critical field of superconductivity Hc2. The broadening of the NMR linewidth below 14K indicates the appearance of the internal field at the Al site, which originates from the antiferromagnetically ordered moments of U 5 f electrons. In the SC state well below μ0Hc2 = 3.4 T, the broadening of the NMR linewidth due to the SC diamagnetism and a decrease in the Knight shift are observed, which are wellunderstood by the framework of spin-singlet superconductivity. In contrast, the Knight shift does not change below Tc(H), and the NMR spectrum is broadened symmetrically in the SC state in the field range of 3 T < μ0H < μ0Hc2. The unusual NMR spectrum near Hc2 suggests that a spatially inhomogeneous SC state such as the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state would be realized. © 2018 The Physical Society of Japan.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Shi2018,

title = {High-field superconductivity on iron chalcogenide FeSe},

author = {A Shi and S Kitagawa and K Ishida and A E Böhmer and C Meingast and T Wolf},

doi = {10.7566/JPSJ.87.065002},

year  = {2018},

date = {2018-01-01},

journal = {Journal of the Physical Society of Japan},

volume = {87},

number = {6},

abstract = {We have performed ac-susceptibility and77Se-NMR measurements on single-crystal FeSe in the field range from 12.5 to 14.75T below 1.6K in order to investigate the superconducting properties of the B phase. Our results show that although superconductivity persists beyond the A–B transition line (H+), the broadening of the 77Se-NMR linewidth arising from the superconducting diamagnetic effect decreases at around H+, suggesting that superconducting character is changed at H+. © 2018 The Physical Society of Japan.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Shi2018b,

title = {Pseudogap behavior of the nuclear spin-lattice relaxation rate in FeSe probed by 77Se-NMR},

author = {A Shi and T Arai and S Kitagawa and T Yamanaka and K Ishida and A E Böhmer and C Meingast and T Wolf and M Hirata and T Sasaki},

doi = {10.7566/JPSJ.87.013704},

year  = {2018},

date = {2018-01-01},

journal = {Journal of the Physical Society of Japan},

volume = {87},

number = {1},

abstract = {We conducted 77Se-nuclear magnetic resonance studies of the iron-based superconductor FeSe in magnetic fields of 0.6 to 19 T to investigate the superconducting and normal-state properties. The nuclear spin-lattice relaxation rate divided by the temperature (T1T)-1 increases below the structural transition temperature Ts but starts to be suppressed below T∗, well above the superconducting transition temperature Tc(H), resulting in a broad maximum of (T1T)-1 at Tp(H). This is similar to the pseudogap behavior in optimally doped cuprate superconductors. Because T∗ and Tp(H) decrease in the same manner as Tc(H) with increasing H, the pseudogap behavior in FeSe is ascribed to superconducting fluctuations, which presumably originate from the theoretically predicted preformed pair above Tc(H). © 2018 The Physical Society of Japan.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kitagawa2017,

title = {Magnetic and superconducting properties of an S -type single-crystal CeCu2Si2 probed by Cu 63 nuclear magnetic resonance and nuclear quadrupole resonance},

author = {S Kitagawa and T Higuchi and M Manago and T Yamanaka and K Ishida and H S Jeevan and C Geibel},

doi = {10.1103/PhysRevB.96.134506},

year  = {2017},

date = {2017-01-01},

journal = {Physical Review B},

volume = {96},

number = {13},

abstract = {We have performed Cu63 nuclear-magnetic-resonance/nuclear-quadrupole-resonance measurements to investigate the magnetic and superconducting (SC) properties on a "superconductivity dominant" (S-type) single crystal of CeCu2Si2. Although the development of antiferromagnetic (AFM) fluctuations down to 1 K indicated that the AFM criticality was close, Korringa behavior was observed below 0.8 K, and no magnetic anomaly was observed above Tc∼0.6 K. These behaviors were expected in S-type CeCu2Si2. The temperature dependence of the nuclear spin-lattice relaxation rate 1/T1 at zero field was almost identical to that in the previous polycrystalline samples down to 130 mK, but the temperature dependence deviated downward below 120 mK. In fact, 1/T1 in the SC state could be fitted with the two-gap s±-wave model rather than the two-gap s++-wave model down to 90 mK. Under magnetic fields, the spin susceptibility in both directions clearly decreased below Tc, which is indicative of the formation of spin-singlet pairing. The residual part of the spin susceptibility was understood by the field-induced residual density of states evaluated from 1/T1T, which was ascribed to the effect of the vortex cores. No magnetic anomaly was observed above the upper critical field Hc2, but the development of AFM fluctuations was observed, indicating that superconductivity was realized in strong AFM fluctuations. © 2017 American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{H.Takahashi_PRBL_2024,

title = {Pressure evolution of the normal- and superconducting-state properties of the line-nodal material CaSb2 revealed by 123Sb nuclear quadrupole resonance},

author = {H Takahashi and S Kitagawa and K Ishida and A Ikeda and S R Saha and S Yonezawa and J Paglione and Y Maeno},

url = {https://arxiv.org/abs/2402.12783},

doi = {10.1103/PhysRevB.109.L100501},

year  = {2024},

date = {2024-03-04},

urldate = {2024-03-04},

journal = {Physical Review B},

volume = {109},

pages = {L100501},

abstract = {CaSb 2 is the Dirac line-nodal material that exhibits a superconducting (SC) transition at 1.7 K. In spite of its conventional SC state at ambient pressure, the transition temperature T c shows a peak structure against hydrostatic pressure. We performed ac magnetic susceptibility and 123 Sb nuclear quadrupole resonance (NQR) measurements on single-crystalline CaSb 2 under pressures up to 2.08 GPa. T c monotonically increased in this pressure region, which is consistent with a previous study. We observed continuous broadening of the NQR spectrum against pressure, which is a sign of unique compression behavior of the lattice. In the normal state, the nuclear spin-lattice relaxation rate 1 / T 1 is proportional to temperature in all pressure values, typical of a metal. However, 1 / T 1 T in the normal state is independent of pressure, indicating that the density of states at the Fermi energy N ( E F ) , which is one of the parameters governing T c , is insensitive to pressure. From these results, we conclude that N ( E F ) does not govern the origin of the enhancement in T c . This is unusual for a weak electron-phonon coupling superconductor. In the SC state, we revealed that the SC gap becomes larger and more isotropic under pressure.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{CaSb2ARPES,

title = {Fermiology of a topological line-nodal compound  CaSb2  and its implication to superconductivity: Angle-resolved photoemission study},

author = {C-Wen Chuang and S Souma and A Moriya and K Nakayama and A Ikeda and M Kawaguchi and K Obata and S Ranjan Saha and H Takahashi and S Kitagawa and K Ishida and K Tanaka and M Kitamura and K Horiba and H Kumigashira and T Takahashi and S Yonezawa and J Paglione and Y Maeno and T Sato},

url = {https://arxiv.org/abs/2211.15083},

doi = {10.1103/PhysRevMaterials.6.104203},

year  = {2022},

date = {2022-10-24},

urldate = {2022-10-24},

journal = {Physical Review Materials},

volume = {6},

issue = {10},

pages = {104203},

abstract = {We performed angle-resolved photoemission spectroscopy with microfocused beam on a topological line-nodal compound CaSb2 which undergoes a superconducting transition at the onset Tc ∼ 1.8 K, to clarify the Fermi-surface topology relevant to the occurrence of superconductivity. We found that a three-dimensional hole pocket at the Γ point is commonly seen for two types of single-crystalline samples fabricated by different growth conditions. On the other hand, the carrier-doping level estimated from the position of the chemical potential was found to be sensitive to the sample fabrication condition. The cylindrical electron pocket at the Y(C) point predicted by the calculations is absent in one of the two samples, despite the fact that both samples commonly show superconductivity with similar Tc's. This suggests a key role of the three-dimensional hole pocket to the occurrence of superconductivity, and further points to an intriguing possibility to control the topological nature of superconductivity by carrier tuning in CaSb2.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{A.Ikeda_PRB_2022,

title = {Quasi-two-dimensional Fermi surface of superconducting line-nodal metal  CaSb2},

author = {A Ikeda and S Ranjan Saha and D Graf and P Saraf and D Sergeevich Sokratov and Y Hu and H Takahashi and S Yamane and A Jayaraj and J Sławińska and M Buongiorno Nardelli and S Yonezawa and Y Maeno and J Paglione},

url = {https://arxiv.org/abs/2206.15346},

doi = {10.1103/PhysRevB.106.075151},

year  = {2022},

date = {2022-08-29},

urldate = {2022-08-29},

journal = {Physical Review B},

volume = {106},

issue = {7},

pages = {075151},

abstract = {We report on the Fermi surfaces and superconducting parameters of CaSb2 single crystals (superconducting below Tc ∼ 1.8 K) grown by the self-flux method. The frequency of de Haas–van Alphen and Shubnikov–de Haas oscillations evidences a quasi-two-dimensional (quasi-2D) Fermi surface, consistent with one of the Fermi surfaces forming Dirac lines predicted by first-principles calculations. Measurements in the superconducting state reveal that CaSb2 is close to a type-I superconductor with the Ginzburg-Landau parameter of around unity. The temperature dependence of the upper critical field Hc2 is well described by a model considering two superconducting bands, and the enhancement of the effective mass estimated from Hc2 (0 K) is consistent with the quasi-2D band observed by the quantum oscillations. Our results indicate that a quasi-2D band forming Dirac lines contributes to the superconductivity in CaSb2.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{H.Takahashi_JPSJ_2021,

title = {S-Wave Superconductivity in the Dirac Line-Nodal Material CaSb2},

author = {H Takahashi and S Kitagawa and K Ishida and M Kawaguchi and A Ikeda and S Yonezawa and Y Maeno},

url = {https://arxiv.org/abs/2105.13614},

doi = {10.7566/JPSJ.90.073702},

year  = {2021},

date = {2021-06-11},

urldate = {2021-06-11},

journal = {Journal of the Physical Society of Japan},

volume = {90},

pages = {073702},

abstract = {We performed 121/123Sb-nuclear quadrupole resonance (NQR) measurements on the superconducting (SC) line-nodal material CaSb2 in order to investigate electronic properties in the normal and SC states from a microscopic point of view. In the normal state, the nuclear spin–lattice relaxation rate 1/T1 for the Sb(1) site, which is responsible for the line-nodal parts, is approximately proportional to temperature, indicating the conventional Fermi liquid state. From comparison with band structure calculations, it is considered that the NQR properties related to the line-nodal character are hidden because the conventional behavior originating from Fermi-surface parts away from the nodes is dominant. In the SC state, a clear coherence peak just below the transition temperature and an exponential decrease at lower temperatures were observed in 1/T1. These results strongly suggest that conventional s-wave superconductivity with a full gap is realized in CaSb2.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Superconducting spin reorientation in spin-triplet multiple superconducting phases of UTe2},

author = {K Kinjo and H Fujibayashi and H Matsumura and F Hori and S Kitagawa and K Ishida and Y Tokunaga and H Sakai and S Kambe and A Nakamura and Y Shimizu and Y Homma and D Li and F Honda and D Aoki},

url = {https://arxiv.org/abs/2307.15784},

doi = {10.1126/sciadv.adg2736},

year  = {2023},

date = {2023-07-28},

urldate = {2023-07-28},

journal = {Science Advances},

volume = {9},

pages = {adg2736},

abstract = {Superconducting (SC) state has spin and orbital degrees of freedom, and spin-triplet superconductivity shows multiple SC phases because of the presence of these degrees of freedom. However, the observation of spin-direction rotation occurring inside the SC state (SC spin rotation) has hardly been reported. Uranium ditelluride, a recently found topological superconductor, exhibits various SC phases under pressure: SC state at ambient pressure (SC1), high-temperature SC state above 0.5 gigapascal (SC2), and low-temperature SC state above 0.5 gigapascal (SC3). We performed nuclear magnetic resonance (NMR) and ac susceptibility measurements on a single-crystal uranium ditelluride. The b axis spin susceptibility remains unchanged in SC2, unlike in SC1, and decreases below the SC2-SC3 transition with spin modulation. These unique properties in SC3 arise from the coexistence of two SC order parameters. Our NMR results confirm spin-triplet superconductivity with SC spin parallel to b axis in SC2 and unveil the remaining of spin degrees of freedom in SC uranium ditelluride.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Gapless fermionic excitation in the antiferromagnetic state of ytterbium zigzag chain},

author = {F Hori and K Kinjo and S Kitagawa and K Ishida and S Mizutani and R Yamamoto and Y Ohmagari and T Onimaru },

url = {https://arxiv.org/abs/2201.07563},

doi = {10.1038/s43246-023-00381-4},

year  = {2023},

date = {2023-07-22},

urldate = {2023-07-22},

journal = {Communications Materials},

volume = {4},

pages = {55},

abstract = {The emergence of charge-neutral fermionic excitations in magnetic systems is one of the unresolved issues in recent condensed matter physics. This type of excitations has been observed in various systems, such as low-dimensional quantum spin liquids, Kondo insulators, and antiferromagnetic insulators. Here, we report the presence of a pronounced gapless spin excitation in the low-temperature antiferromagnetic state of YbCuS2 semiconductor, where trivalent ytterbium atoms form a zigzag chain structure. We confirm the presence of this gapless excitations by a combination of experimental probes, namely 63/65Cu-nuclear magnetic resonance and nuclear quadrupole resonance, as well as specific heat measurements, revealing a linear low-temperature behavior of both the nuclear spin-lattice relaxation rate 1/T1 and the specific heat. This system provides a platform to investigate the origin of gapless excitations in spin chains and the relationship between emergent fermionic excitations and frustration.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{S.Kitagawa_PRB_2022,

title = {Enhancement of charge-neutral fermionic excitations near the spin-flop transition in the magnetic Kondo material YbIr3Si7},

author = {S Kitagawa and T Kobayashi and F Hori and K Ishida and A H Nevidomskyy and L Qian and E Morosan},

url = {https://doi.org/10.1103/PhysRevB.106.L100405

https://arxiv.org/abs/2209.10844},

doi = {10.1103/PhysRevB.106.L100405},

year  = {2022},

date = {2022-09-16},

urldate = {2022-09-16},

journal = {Physical Review B},

volume = {106},

pages = {L100405},

abstract = {The new Kondo material YbIr3Si7, similar to other Kondo insulators, has been reported to exhibit charge-neutral fermionic excitations through measurements of specific heat and thermal conductivity at low temperatures. 

We performed 29Si−NMR on YbIr3Si7 to investigate the magnetic response of charge-neutral fermions from a microscopic perspective. In low magnetic fields parallel to the c axis, a single NMR peak in the paramagnetic state splits into three peaks below TN. 

In contrast, only a slight shift of the single NMR peak was observed in high magnetic fields. This spectral change as a function of the c

-axis magnetic field is interpreted as a spin-flop transition, at which the magnetic moments oriented along the c axis antiferromagnetic (AFM-I) phase are rotated to the ab plane with a ferromagnetic component along the c-axis (AFM-II phase). In the vicinity of the spin-flop magnetic field 

HM, the nuclear spin-lattice relaxation rate 1/T1 was found to be proportional to temperature at low temperatures, indicating the existence of charge-neutral fermions. Furthermore, a peak of 1/T1 versus the c-axis magnetic field suggests that the charge-neutral fermions in YbIr3Si7 are closely related to its magnetic properties. Our findings shed light on the origin of charge-neutral fermions in insulators.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Impurity-Robust Bulk Gapless Excitation in the Yb-Based Zigzag Chain Compound YbCuS2},

author = {F Hori and K Kinjo and S Kitagawa and K Ishida and Y Ohmagari and T Onimaru},

doi = {10.1088/1742-6596/2164/1/012027},

year  = {2022},

date = {2022-03-17},

journal = {J. Phys.: Conf. Ser.},

volume = {2164},

pages = {012027},

abstract = {We have performed 63Cu-nuclear quadrupole resonance (NQR) measurements using a lump sample of the Yb zigzag-chain compound YbCuS2 with a small surface area to investigate the sample dependence of low-temperature magnetic properties in YbCuS2 by comparing with the previous study with different powdered sample. The line width of NQR signals in the present lump sample is larger than that in the previous powdered sample. In addition, the transition temperature TN ∼ 0.92 K in the present lump sample is lower than that in the previous powdered sample (∼ 0.95 K). These results suggest that the quality of the present lump sample is worse than that of the previous powdered sample. However, the T-linear behavior of the nuclear spin-lattice relaxation rate 1/T1 was observed below 0.5 K and the value of 1/T1T in both samples is almost the same even though the sample quality and sample geometry are different. This suggests that T-linear behavior in 1/T1 arises from the impurity-robust bulk gapless excitation inherent in YbCuS2 rather than from sample issues such as the sample quality or geometry.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{S.Ogata_PRL_2023,

title = {Parity transition of spin-singlet superconductivity using sub-lattice degrees of freedom},

author = {S Ogata and S Kitagawa and K Kinjo and K Ishida and M Brando and E Hassinger and C Geibel and S Khim},

url = {https://arxiv.org/abs/2304.10032},

doi = {10.1103/PhysRevLett.130.166001},

year  = {2023},

date = {2023-04-19},

urldate = {2023-04-19},

journal = {Physical Review Letters},

volume = {130},

pages = {166001 },

abstract = {Recently, a superconducting (SC) transition from low-field (LF) to high-field (HF) SC states was reported in CeRh2As2, indicating the existence of multiple SC states. It has been theoretically noted that the existence of two Ce sites in the unit cell, the so-called sub-lattice degrees of freedom owing to the local inversion symmetry breaking at the Ce sites, can lead to the appearance of multiple SC phases even under an interaction inducing spin-singlet superconductivity. CeRh2As2 is considered as the first example of multiple SC phases owing to this sub-lattice degree of freedom. However, microscopic information about the SC states has not yet been reported. In this study, we measured the SC spin susceptibility at two crystallographically inequivalent As sites using nuclear magnetic resonance for various magnetic fields. Our experimental results strongly indicate a spin-singlet state in both SC phases. In addition, the antiferromagnetic phase, which appears within the SC phase, only coexists with the LF SC phase; there is no sign of magnetic ordering in the HF SC phase. The present work reveals unique SC properties originating from the locally noncentrosymmetric characteristics. },

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{S.Kitagawa_PRL_2022,

title = {Observation of Antiferromagnetic Order as Odd-Parity Multipoles inside the Superconducting Phase in  CeRh2As2},

author = {M Kibune and S Kitagawa and K Kinjo and S Ogata and M Manago and T Taniguchi and K Ishida and M Brando and E Hassinger and H Rosner and C Geibel and S Khim},

url = {https://arxiv.org/abs/2112.07081},

doi = {10.1103/PhysRevLett.128.057002},

year  = {2022},

date = {2022-02-03},

urldate = {2022-02-03},

journal = {Physical Review Letters},

volume = {128},

number = {05},

pages = {057002},

abstract = {Spatial inversion symmetry in crystal structures is closely related to the superconducting (SC) and magnetic properties of materials. Recently, several theoretical proposals that predict various interesting phenomena caused by the breaking of the local inversion symmetry have been presented. However, experimental validation has not yet progressed owing to the lack of model materials. Here we present evidence for antiferromagnetic (AFM) order in CeRh2As2 (SC transition temperature TSC ∼ 0.37 K ), wherein the Ce site breaks the local inversion symmetry. The evidence is based on the observation of different extents of broadening of the nuclear quadrupole resonance spectrum at two crystallographically inequivalent As sites. This AFM ordering breaks the inversion symmetry of this system, resulting in the activation of an odd-parity magnetic multipole. Moreover, the onset of antiferromagnetism TN within an SC phase, with TN < TSC , is quite unusual in systems wherein superconductivity coexists or competes with magnetism. Our observations show that CeRh2As2 is a promising system to study how the absence of local inversion symmetry induces or influences unconventional magnetic and SC states, as well as their interaction.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Superconducting spin reorientation in spin-triplet multiple superconducting phases of UTe2},

author = {K Kinjo and H Fujibayashi and H Matsumura and F Hori and S Kitagawa and K Ishida and Y Tokunaga and H Sakai and S Kambe and A Nakamura and Y Shimizu and Y Homma and D Li and F Honda and D Aoki},

url = {https://arxiv.org/abs/2307.15784},

doi = {10.1126/sciadv.adg2736},

year  = {2023},

date = {2023-07-28},

urldate = {2023-07-28},

journal = {Science Advances},

volume = {9},

pages = {adg2736},

abstract = {Superconducting (SC) state has spin and orbital degrees of freedom, and spin-triplet superconductivity shows multiple SC phases because of the presence of these degrees of freedom. However, the observation of spin-direction rotation occurring inside the SC state (SC spin rotation) has hardly been reported. Uranium ditelluride, a recently found topological superconductor, exhibits various SC phases under pressure: SC state at ambient pressure (SC1), high-temperature SC state above 0.5 gigapascal (SC2), and low-temperature SC state above 0.5 gigapascal (SC3). We performed nuclear magnetic resonance (NMR) and ac susceptibility measurements on a single-crystal uranium ditelluride. The b axis spin susceptibility remains unchanged in SC2, unlike in SC1, and decreases below the SC2-SC3 transition with spin modulation. These unique properties in SC3 arise from the coexistence of two SC order parameters. Our NMR results confirm spin-triplet superconductivity with SC spin parallel to b axis in SC2 and unveil the remaining of spin degrees of freedom in SC uranium ditelluride.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{H.Matsumura_JPSJ_2023,

title = {Large Reduction in the a-axis Knight Shift on UTe2 with Tc = 2.1 K},

author = {H Matsumura and H Fujibayashi and K Kinjo and S Kitagawa and K Ishida and Y Tokunaga and H Sakai and S Kambe and A Nakamura and Y Shimizu and Y Homma and D Li and F Honda and D Aoki },

url = {https://arxiv.org/abs/2305.01200},

doi = {10.7566/JPSJ.92.063701},

year  = {2023},

date = {2023-05-16},

urldate = {2023-05-16},

journal = {Journal of the Physical Society of Japan},

volume = {92},

pages = {063701},

abstract = {Spin susceptibility in the superconducting (SC) state was measured in the higher-quality sample of uranium-based superconductor UTe2 by using Knight-shift measurements for a magnetic field H along all three crystalline axes. In the higher-quality sample, the SC transition temperature Tc is about 2.1 K, and the residual electronic term in the specific heat is almost zero. The NMR linewidth becomes narrower and is almost half of that in the previous sample with Tc ∼ 1.6 K when H || a and c. Although the Knight-shift behavior was not so different from the previous results for H || b, and c, a large reduction in Knight shift along the a axis was observed, in contrast with the previous a-axis Knight shift result. We discuss the origin of the difference between the previous and present results, and the possible SC state derived from the present results.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}