論文リスト

@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{K.Jenni_PRB2021,

title = {Neutron scattering studies on spin fluctuations in  Sr2RuO4},

author = {K Jenni and S Kunkemöller and P. Steffens and Y Sidis and R Bewley and Z Q Mao and Y Maeno and M Braden},

doi = {10.1103/PhysRevB.103.104511},

year  = {2021},

date = {2021-03-18},

urldate = {2021-03-18},

journal = {Physical Review B},

volume = {103},

number = {10},

pages = {104511},

abstract = {The magnetic excitations in Sr2RuO4 are studied by polarized and unpolarized neutron scattering experiments as a function of temperature. At the scattering vector of the Fermi-surface nesting with a half-integer out-of-plane component, there is no evidence for the appearance of a resonance excitation in the superconducting phase. The body of existing data indicates weakening of the scattered intensity in the nesting spectrum to occur at very low energies. The nesting signal persists up to 290 K but is strongly reduced. In contrast, a quasiferromagnetic contribution maintains its strength and still exhibits a finite width in momentum space.},

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{SRO_heatcapacity_uniax,

title = {High-sensitivity heat-capacity measurements on Sr2RuO4 under uniaxial pressure},

author = {Y-S Li and N Kikugawa and D A Sokolov and F Jerzembeck and A S Gibbs and Y Maeno and C W Hicks and J Schmalian and M Nicklas and A P Mackenzie},

editor = {Z Fisk },

doi = {10.1073/pnas.2020492118},

year  = {2021},

date = {2021-03-09},

urldate = {2021-03-09},

journal = {PNAS},

volume = {118},

number = {10},

abstract = {A key question regarding the unconventional superconductivity of Sr2RuO4 remains whether the order parameter is single- or two-component. Under a hypothesis of two-component superconductivity, uniaxial pressure is expected to lift their degeneracy, resulting in a split transition. The most direct and fundamental probe of a split transition is heat capacity. Here, we report measurement of heat capacity of samples subject to large and highly homogeneous uniaxial pressure. We place an upper limit on the heat-capacity signature of any second transition of a few percent of that of the primary superconducting transition. The normalized jump in heat capacity, ΔC/C, grows smoothly as a function of uniaxial pressure, favoring order parameters which are allowed to maximize in the same part of the Brillouin zone as the well-studied van Hove singularity. Thanks to the high precision of our measurements, these findings place stringent constraints on theories of the superconductivity of Sr2RuO4.},

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{mattoni2020role,

title = {Role of local temperature in the current-driven metal–insulator transition of Ca_{2}RuO_{4}},

author = {G Mattoni and S Yonezawa and F Nakamura and Y Maeno},

url = {https://arxiv.org/abs/2007.06885},

doi = {10.1103/PhysRevMaterials.4.114414},

year  = {2020},

date = {2020-11-17},

journal = {Physical Review Materials},

volume = {4},

number = {114414},

abstract = {It was recently reported that a continuous electric current is a powerful control parameter to trigger changes in the electronic structure and metal–insulator transitions (MITs) in Ca2RuO4. However, the spatial evolution of the MIT and the implications of the unavoidable Joule heating have not been clarified yet, often hindered by the difficulty to assess the local sample temperature. In this work, we perform infrared thermal imaging on single-crystal Ca2RuO4 while controlling the MIT by electric current. The change in emissivity at the phase transition allows us to monitor the gradual formation and expansion of the metallic phase upon increasing current. Our local temperature measurements indicate that, within our experimental resolution, the MIT always occurs at the same local transition temperatures, irrespective of whether it is driven by temperature or current. Our results highlight the importance of local heating, phase coexistence, and microscale inhomogeneity when studying strongly correlated materials under the flow of electric current.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{SRO_ultrasound,

title = {Ultrasound evidence for a two-component superconducting order parameter in Sr2RuO4},

author = {S. Benhabib and C. Lupien and I. Paul and L. Berges and M. Dion and M. Nardone and A. Zitouni and Z. Q. Mao and Y. Maeno and A. Georges and L. Taillefer and C. Proust},

doi = {10.1038/s41567-020-01090-2},

year  = {2020},

date = {2020-10-21},

journal = {Nature Physics},

volume = {17},

pages = {194-198},

abstract = {The quasi-two-dimensional metal Sr2RuO4 is one of the best characterized unconventional superconductors, yet the nature of its superconducting order parameter is still under debate. This information is crucial to determine the pairing mechanism of Cooper pairs. Here we use ultrasound velocity to probe the superconducting state of Sr2RuO4. This thermodynamic probe is sensitive to the symmetry of the material, and therefore, it can help in identifying the symmetry of the superconducting order parameter. Indeed, we observe a sharp jump in the shear elastic constant c66 as the temperature is increased across the superconducting transition. This directly implies that the superconducting order parameter is of a two-component nature. On the basis of symmetry arguments and given the other known properties of Sr2RuO4, we discuss which states are compatible with this requirement and propose that the two-component order parameter {dxz; dyz} is the most likely candidate.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Ivan_NatCommun_2020,

title = {Uniaxial-strain control of nematic superconductivity in SrxBi2Se3},

author = {I Kostylev and S Yonezawa and Z Wang and Y Ando and Y Maeno },

doi = {10.1038/s41467-020-17913-y},

year  = {2020},

date = {2020-08-24},

journal = {Nature Communications},

volume = {11},

pages = {4152},

abstract = {Nematic states are characterized by rotational symmetry breaking without translational ordering. Recently, nematic superconductivity, in which the superconducting gap spontaneously lifts the rotational symmetry of the lattice, has been discovered. In nematic superconductivity, multiple superconducting domains with different nematic orientations can exist, and these domains can be controlled by a conjugate external stimulus. Domain engineering is quite common in magnets but has not been achieved in superconductors. Here, we report control of the nematic superconductivity and their domains of SrxBi2Se3, through externally-applied uniaxial stress. The suppression of subdomains indicates that it is the Δ4y state that is most favoured under compression along the basal Bi-Bi bonds. This fact allows us to determine the coupling parameter between the nematicity and lattice distortion. These results provide an inevitable step towards microscopic understanding and future utilization of the unique topological nematic superconductivity.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Maeno_FC_2020,

title = {Three related topics on the periodic tables of elements},

author = {Y Maeno and K Hagino and T Ishiguro },

doi = {10.1007/s10698-020-09387-z},

year  = {2020},

date = {2020-08-18},

journal = {Foundations of Chemistry},

abstract = {A large variety of periodic tables of the chemical elements have been proposed. It was Mendeleev who proposed a periodic table based on the extensive periodic law and predicted a number of unknown elements at that time. The periodic table currently used worldwide is of a long form pioneered by Werner in 1905. As the first topic, we describe the work of Pfeiffer (Naturwiss. 8:984–991, 1920), who refined Werner’s work and rearranged the rare-earth elements in a separate table below the main table for convenience. Today’s widely used periodic table essentially inherits Pfeiffer’s arrangements. Although long-form tables more precisely represent electron orbitals around a nucleus, they lose some of the features of Mendeleev’s short-form table to express similarities of chemical properties of elements when forming compounds. As the second topic, we compare various three-dimensional (3D) helical periodic tables that resolve some of the shortcomings of the long-form periodic tables in this respect. In particular, we explain how the 3D periodic table “Elementouch” (Maeno in Periodic-table-of-the-elements stationery. Design No. 1149493, Japan Patent Office. https://www.j-platpat.inpit.go.jp/d0000, 2001), which combines the s- and p-blocks into one tube, can recover features of Mendeleev’s periodic law. Finally we introduce a topic on the recently proposed nuclear periodic table based on the proton magic numbers (Hagino and Maeno in Found Chem 22:267–273, 2020). Here, the nuclear shell structure leads to a new arrangement of the elements with the proton magic-number nuclei treated like noble-gas atoms. We show that the resulting alignments of the elements in both the atomic and nuclear periodic tables are common over about two thirds of the tables because of a fortuitous coincidence in their magic numbers.},

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{Ikeda_PRB_2020,

title = {Penetration depth and gap structure in the antiperovskite oxide superconductor Sr_{3−x}SnO revealed by μSR},

author = {A Ikeda and Z Guguchia and M Oudah and S Koibuchi and S Yonezawa and D Das and T Shiroka and H Luetkens and Y Maeno},

url = {http://hdl.handle.net/2433/250921

https://arxiv.org/abs/2004.06993},

doi = {10.1103/PhysRevB.101.174503},

year  = {2020},

date = {2020-05-04},

journal = {Physical Review B},

volume = {101},

number = {17},

pages = {174503},

abstract = {We report a μSR study on the antiperovskite oxide superconductor Sr3−xSnO. By using transverse-field μSR, we observed an increase of the muon relaxation rate upon cooling below the superconducting transition temperature Tc =5.4 K, evidencing bulk superconductivity. 

The exponential temperature dependence of the relaxation rate σ at low temperatures suggests a fully gapped superconducting state. We evaluated the zero-temperature penetration depth λ(0) ∝ 1/√σ(0) to be around 320–1020 nm. Such a large value is consistent with the picture of a doped Dirac semimetal. 

Moreover, we established that the ratio Tc/λ(0)−2 is larger than those of ordinary superconductors and is comparable to those of unconventional superconductors. The relatively high Tc for small carrier density may hint at an unconventional pairing mechanism beyond the ordinary phonon-mediated pairing. In addition, zero-field μSR did not provide evidence of broken time-reversal symmetry in the superconducting state. These features are consistent with the theoretically proposed topological superconducting state in Sr3−xSnO, as well as with ordinary s-wave superconductivity.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{mattoni2020diamagnetic,

title = {Diamagnetic-like response from localized heating of a paramagnetic material},

author = {G Mattoni and S Yonezawa and Y Maeno},

url = {https://arxiv.org/abs/2004.04570},

doi = {10.1063/5.0006098},

year  = {2020},

date = {2020-04-30},

journal = {Applied Physics Letters},

volume = {116},

number = {17},

pages = {172405},

publisher = {AIP Publishing LLC},

abstract = {In the search of material properties out-of-equilibrium, the non-equilibrium steady states induced by electric current are an appealing research direction where unconventional states may emerge. However, the unavoidable Joule heating caused by flowing current calls for the development of new measurement protocols, with a particular attention to the physical properties of the background materials involved. Here, we demonstrate that localised heating can give rise to a large, spurious diamagnetic-like signal. This occurs due to the local reduction of the background magnetisation caused by the heated sample, provided that the background material has a Curie-like susceptibility. Our experimental results, along with numerical calculations, constitute an important building block for performing accurate magnetic measurements under the flow of electric current.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Ikeda_PRR_2020,

title = {Superconductivity in the nonsymmorphic line-nodal compound  CaSb_{2}},

author = {A Ikeda and M Kawaguchi and S Koibuchi and T Hashimoto and T Kawakami and S Yonezawa and M Sato and Y Maeno},

url = {https://arxiv.org/abs/2003.12800},

doi = {10.1103/PhysRevMaterials.4.041801},

year  = {2020},

date = {2020-04-13},

journal = {Physical Review Materials},

volume = {4},

number = {4},

pages = {041801(R)},

abstract = {We found superconductivity in CaSb2 with the transition temperature of 1.7 K by means of electrical-resistivity, magnetic-susceptibility, and specific-heat measurements. This material crystallizes in a nonsymmorphic structure and is predicted to have multiple Dirac nodal lines in the bulk electronic band structure protected by symmetry even in the presence of spin-orbit coupling. We discuss a possible topological superconductivity for the quasi-two-dimensional band originating mainly from one of the antimony sites.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Yasui2020,

title = {Spontaneous emergence of Josephson junctions in homogeneous rings of single-crystal Sr_{2}RuO_{4}},

author = {Y Yasui and K Lahabi and V F Becerra and R Fermin and M S Anwar and S Yonezawa and T Terashima and M V Milošević and J Aarts and Y Maeno},

url = {https://arxiv.org/abs/2002.06728},

doi = {10.1038/s41535-020-0223-7},

year  = {2020},

date = {2020-04-09},

journal = {npj Quantum Materials},

volume = {5},

number = {1},

abstract = {The chiral p-wave order parameter in Sr2RuO4 would make it a special case amongst the unconventional superconductors. A consequence of this symmetry is the possible existence of superconducting domains of opposite chirality. At the boundary of such domains, the locally suppressed condensate can produce an intrinsic Josephson junction. Here, we provide evidence of such junctions using mesoscopic rings, structured from Sr2RuO4 single crystals. Our order parameter simulations predict such rings to host stable domain walls across their arms. This is verified with transport experiments on loops, with a sharp transition at 1.5 K, which show distinct critical current oscillations with periodicity corresponding to the flux quantum. In contrast, loops with broadened transitions at around 3 K are void of such junctions and show standard Little–Parks oscillations. Our analysis demonstrates the junctions are of intrinsic origin and makes a compelling case for the existence of superconducting domains. © 2020, The Author(s).},

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{Ikeda2019,

title = {Negative ionic states of tin in the oxide superconductor Sr_{3-x}SnO revealed by Mössbauer spectroscopy},

author = {A Ikeda and S Koibuchi and S Kitao and M Oudah and S Yonezawa and M Seto and Y Maeno},

url = {http://hdl.handle.net/2433/250418

https://arxiv.org/abs/1911.12934},

doi = {10.1103/PhysRevB.100.245145},

year  = {2019},

date = {2019-12-24},

journal = {Physical Review B},

volume = {10},

number = {24},

pages = {245145},

abstract = {We report the temperature variation of the Sn119-Mössbauer spectra of the antiperovskite (inverse perovskite) oxide superconductor Sr3-xSnO. Both superconductive (Sr deficient) and nonsuperconductive (nearly stoichiometric) samples exhibit major γ-ray absorption with an isomer shift similar to that of Mg2Sn. This fact shows that Sr3-xSnO contains the metallic anion Sn4-, which is rare, especially among oxides. In both samples, we observed another γ-ray absorption with a larger isomer shift, indicating that there is another ionic state of Sn with a higher oxidation number. The temperature dependence of the absorption intensities reveals that the Sn ions exhibiting larger isomer shifts have a lower energy of the local vibration. The larger isomer shift and lower vibration energy are consistent with the values estimated from the first-principles calculations for hypothetical structures with various Sr-deficiency arrangements. Therefore, we conclude that the additional γ-ray absorptions originate from the Sn atoms neighboring the Sr deficiency. © 2019 American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Anwar2019,

title = {Anomalous anisotropic behaviour of spin-triplet proximity effect in Au/SrRuO_{3}/Sr_{2}RuO_{4} junctions},

author = {M S Anwar and M Kunieda and R Ishiguro and S R Lee and C Sow and J W A Robinson and S Yonezawa and T W Noh and Y Maeno},

doi = {10.1038/s41598-019-52003-0},

year  = {2019},

date = {2019-11-01},

journal = {Scientific Reports},

volume = {9},

number = {1},

abstract = {Spin-polarized supercurrents can be generated with magnetic inhomogeneity at a ferromagnet/spin-singlet-superconductor interface. In such systems, complex magnetic inhomogeneity makes it difficult to functionalise the spin-polarized supercurrents. However, spin-polarized supercurrents in ferromagnet/spin-triplet-superconductor junctions can be controlled by the angle between magnetization and spin of Copper pairs (d-vector), that can effectively be utilized in developing of a field of research known as superconducting spintronics. Recently, we found induction of spin-triplet correlation into a ferromagnet SrRuO3 epitaxially deposited on a spin-triplet superconductor Sr2RuO4, without any electronic spin-flip scattering. Here, we present systematic magnetic field dependence of the proximity effect in Au/SrRuO3/Sr2RuO4 junctions. It is found that induced triplet correlations exhibit strongly anisotropic field response. Such behaviour is attributed to the rotation of the d-vector of Sr2RuO4. This anisotropic behaviour is in contrast with the vortex dynamic. Our results will stimulate study of interaction between ferromagnetism and unconventional superconductivity. © 2019, The Author(s).},

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{Kashiwaya2019,

title = {Time-reversal invariant superconductivity of Sr_{2}RuO_{4} revealed by Josephson effects},

author = {S Kashiwaya and K Saitoh and H Kashiwaya and M Koyanagi and M Sato and K Yada and Y Tanaka and Y Maeno},

doi = {10.1103/PhysRevB.100.094530},

year  = {2019},

date = {2019-09-25},

journal = {Physical Review B},

volume = {100},

number = {9},

abstract = {Sr2RuO4 is one of the most promising candidates of a topological superconductor with broken time-reversal symmetry because a number of experiments have revealed evidence for a spin-triplet chiral p-wave superconductivity. To clarify the time-reversal symmetry of Sr2RuO4, we introduce a test that examines the invariance of the Josephson critical current under the inversion of both the current and magnetic fields, in contrast to the detection of a spontaneous magnetic field employed in past experiments. Analyses of the transport properties of the planar and corner Josephson junctions formed between Sr2RuO4 and Nb reveal the time-reversal invariant superconductivity, most probably helical p-wave symmetry, of Sr2RuO4. This state corresponds to a yet-to-be confirmed topological crystalline superconductivity that can host two Majorana edge modes at the surface protected by crystalline mirror symmetry. © 2019 American Physical Society.},

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{Anwar2019b,

title = {Observation of superconducting gap spectra of long-range proximity effect in Au/SrTiO_{3}/SrRuO_{3}/Sr_{2}RuO_{4} tunnel junctions},

author = {M S Anwar and M Kunieda and R Ishiguro and S R Lee and L A B O Olthof and J W A Robinson and S Yonezawa and T W Noh and Y Maeno},

doi = {10.1103/PhysRevB.100.024516},

year  = {2019},

date = {2019-07-24},

journal = {Physical Review B},

volume = {100},

number = {2},

abstract = {We observe an unconventional superconducting minigap induced into a ferromagnet SrRuO3 from a spin-triplet superconductor Sr2RuO4 using an Au/SrTiO3/SrRuO3/Sr2RuO4 tunnel junction. Voltage bias differential conductance of the tunnel junctions exhibits V-shaped gap features around zero bias, corresponding to a decrease in the density of states with an opening of a superconducting minigap in SrRuO3. Observation of a minigap at the surface of a 15-nm-thick SrRuO3 layer confirms the spin-triplet nature of induced superconductivity. The shape and temperature dependence of the gap features in the differential conductance suggest that the even-frequency p-wave correlations dominate over odd-frequency s-wave correlations. Theoretical calculations support this p-wave scenario. Our work provides the density-of-states proof for an induced minigap in a ferromagnet, and it significantly advanced our understanding of the spin-triplet proximity effect between unconventional superconductors and ferromagnets. © 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{Sow2019,

title = {In situ control of diamagnetism by electric current in Ca_{3}(Ru_{1-x}Ti_{x})_{2}O_{7}},

author = {C Sow and R Numasaki and G Mattoni and S Yonezawa and N Kikugawa and S Uji and Y Maeno},

url = {https://arxiv.org/abs/1902.02515},

doi = {10.1103/PhysRevLett.122.196602},

year  = {2019},

date = {2019-05-17},

journal = {Physical Review Letters},

volume = {122},

number = {19},

abstract = {Nonequilibrium steady state conditions induced by a dc current can alter the physical properties of strongly correlated electron systems. In this regard, it was recently shown that dc current can trigger novel electronic states, such as current-induced diamagnetism, which cannot be realized in equilibrium conditions. However, reversible control of diamagnetism has not been achieved yet. Here, we demonstrate reversible in situ control between a Mott insulating state and a diamagnetic semimetal-like state by a dc current in the Ti-substituted bilayer ruthenate Ca3(Ru1-xTix)2O7 (x=0.5%). By performing simultaneous magnetic and resistive measurements, we map out the temperature vs current-density phase diagram in the nonequilibrium steady state of this material. The present results open up the possibility of creating novel electronic states in a variety of strongly correlated electron systems under dc current. © 2019 American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Lee2019,

title = {Strong spin-phonon coupling unveiled by coherent phonon oscillations in Ca_{2}RuO_{4}},

author = {M -C Lee and C H Kim and I Kwak and C W Seo and C Sohn and F Nakamura and C Sow and Y Maeno and E -A Kim and T W Noh and K W Kim},

doi = {10.1103/PhysRevB.99.144306},

year  = {2019},

date = {2019-04-15},

journal = {Physical Review B},

volume = {99},

number = {14},

abstract = {We utilize near-infrared femtosecond pulses to investigate coherent phonon oscillations of Ca2RuO4. The coherent Ag phonon mode of the lowest frequency changes abruptly not only its amplitude but also the oscillation phase as the spin order develops. In addition, the phonon mode shows a redshift entering the magnetically ordered state, which indicates a spin-phonon coupling in the system. Density functional theory calculations reveal that the Ag oscillations result in octahedral tilting distortions, which are exactly in sync with the lattice deformation driven by the magnetic ordering. We suggest that the structural distortions by the spin-phonon coupling can induce the unusual oscillation phase shift between impulsive and displacive type oscillations. © 2019 American Physical Society.},

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{Zhang2019,

title = {Nano-Resolved Current-Induced Insulator-Metal Transition in the Mott Insulator Ca_{2}RuO_{4}},

author = {J Zhang and A S McLeod and Q Han and X Chen and H A Bechtel and Z Yao and S N Gilbert Corder and T Ciavatti and T H Tao and M Aronson and G L Carr and M C Martin and C Sow and S Yonezawa and F Nakamura and I Terasaki and D N Basov and A J Millis and Y Maeno and M Liu},

doi = {10.1103/PhysRevX.9.011032},

year  = {2019},

date = {2019-02-15},

journal = {Physical Review X},

volume = {9},

number = {1},

abstract = {The Mott insulator Ca2RuO4 is the subject of much recent attention following reports of emergent nonequilibrium steady states driven by applied electric fields or currents. In this paper, we carry out infrared nano-imaging and optical-microscopy measurements on bulk single crystal Ca2RuO4 under conditions of steady current flow to obtain insight into the current-driven insulator-to-metal transition. We observe macroscopic growth of the current-induced metallic phase, with nucleation regions for metal and insulator phases determined by the polarity of the current flow. A remarkable metal-insulator-metal microstripe pattern is observed at the phase front separating metal and insulator phases. The microstripes have orientations tied uniquely to the crystallographic axes, implying a strong coupling of the electronic transition to lattice degrees of freedom. Theoretical modeling further illustrates the importance of the current density and confirms a submicron-thick surface metallic layer at the phase front of the bulk metallic phase. Our work confirms that the electrically induced metallic phase is nonfilamentary and is not driven by Joule heating, revealing remarkable new characteristics of electrically induced insulator-metal transitions occurring in functional correlated oxides. © 2019 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the »https://creativecommons.org/licenses/by/4.0/» Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Oudah2019,

title = {Evolution of Superconductivity with Sr-Deficiency in Antiperovskite Oxide Sr_{3−x}SnO},

author = {M Oudah and J N Hausmann and S Kitao and A Ikeda and S Yonezawa and M Seto and Y Maeno},

doi = {10.1038/s41598-018-38403-8},

year  = {2019},

date = {2019-02-12},

journal = {Scientific Reports},

volume = {9},

number = {1},

pages = {1831},

abstract = {Bulk superconductivity was recently reported in the antiperovskite oxide Sr 3−x SnO, with a possibility of hosting topological superconductivity. We investigated the evolution of superconducting properties such as the transition temperature T c and the size of the diamagnetic signal, as well as normal-state electronic and crystalline properties, with varying the nominal Sr deficiency x 0 . Polycrystalline Sr 3−x SnO was obtained up to x 0 = 0:6 with a small amount of SrO impurities. The amount of impurities increases for x 0 > 0.6, suggesting phase instability for high deficiency. Mössbauer spectroscopy reveals an unusual Sn 4− ionic state in both stoichiometric and deficient samples. By objectively analyzing superconducting diamagnetism data obtained from a large number of samples, we conclude that the optimal x 0 lies in the range 0.5 < x 0 < 0.6. In all superconducting samples, two superconducting phases appear concurrently that originate from Sr 3−x SnO but with varying intensities. These results clarify the Sr deficiency dependence of the normal and superconducting properties of the antiperovskite oxide Sr 3−x SnO will ignite future work on this class of materials. © 2019, The Author(s).},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Akebi2019,

title = {Low-energy electron-mode couplings in the surface bands of Sr_{2}RuO_{4} revealed by laser-based angle-resolved photoemission spectroscopy},

author = {S Akebi and T Kondo and M Nakayama and K Kuroda and S Kunisada and H Taniguchi and Y Maeno and S Shin},

doi = {10.1103/PhysRevB.99.081108},

year  = {2019},

date = {2019-02-07},

journal = {Physical Review B},

volume = {99},

number = {8},

abstract = {We use angle-resolved photoemission spectroscopy with a 7-eV laser, which is capable of ultrahigh energy and momentum resolutions, and investigate bosonic mode couplings in the surface bands of Sr2RuO4. Significantly, our measurements reveal pronounced kink structures at very low binding energies (∼8 and ∼15 meV), which have a strong variation in the renormalization strength among multiple bands derived from the Ru4d orbitals. Neutron scattering has observed phonon modes with the lowest energy scale around 15 meV, which thus could be the main source for the ∼15-meV kink. In contrast, the significant coupling at the lower energy (∼8 meV) is attributed to magnetic excitations, which are reported to be peaked in density of states around 10 meV. The ∼8-meV kink is found to be strongest in the two-dimensional 4dxy-derived band, implying its mechanism to be the electron coupling to ferromagnetic spin fluctuations. © 2019 American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Steffens2019,

title = {Spin Fluctuations in Sr_{2}RuO_{4} from Polarized Neutron Scattering: Implications for Superconductivity},

author = {P Steffens and Y Sidis and J Kulda and Z Q Mao and Y Maeno and I I Mazin and M Braden},

doi = {10.1103/PhysRevLett.122.047004},

year  = {2019},

date = {2019-02-01},

journal = {Physical Review Letters},

volume = {122},

number = {4},

abstract = {Triplet pairing in Sr2RuO4 was initially suggested based on the hypothesis of strong ferromagnetic spin fluctuations. Using polarized inelastic neutron scattering, we accurately determine the full spectrum of spin fluctuations in Sr2RuO4. Besides the well-studied incommensurate magnetic fluctuations, we do find a sizable quasiferromagnetic signal, quantitatively consistent with all macroscopic and microscopic probes. We use this result to address the possibility of magnetically driven triplet superconductivity in Sr2RuO4. We conclude that, even though the quasiferromagnetic signal is stronger and sharper than previously anticipated, spin fluctuations alone are not enough to generate a triplet state strengthening the need for additional interactions or an alternative pairing scenario. © 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{Kostylev2019,

title = {Piezoelectric-based uniaxial strain cell with high strain throughput and homogeneity},

author = {I Kostylev and S Yonezawa and Y Maeno},

doi = {10.1063/1.5063729},

year  = {2019},

date = {2019-01-16},

journal = {Journal of Applied Physics},

volume = {125},

number = {8},

abstract = {We seek for novel electronic phenomena by using external strain engineering, namely, by the application of external uniaxial strain. A piezoelectric-based uniaxial strain cell has been recently developed by Hicks et al. and successfully utilized for studies of various unconventional superconductors. Here, we propose a modified design that minimizes effects originating from the asymmetry of the strain cell design, in particular with minimal bending moments, by placing the sample at the axis of mirror symmetry of the device to balance out the reactive forces. Results of finite element analyses indicate that placing the sample offset to the axis of the mirror symmetry indeed causes an increase of strain inhomogeneity up to 10%, both at room temperature and at 4.2 K. Furthermore, we found that this bending reduces the overall achievable strain down to 30% of its maximal value without the bending. We also experimentally demonstrate the potential of our constructed device by tracking the displacement of the anvils of the strain cell with a parallel plate capacitor. The full range of motion of the piezo-stacks were demonstrated at room temperature. With a sample and at 0.9 K, we achieved a uniaxial strain of - 1.5%, which is enough to induce electronic change in typical materials. © 2019 Author(s).},

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{Ikeda2018752,

title = {Theoretical band structure of the superconducting antiperovskite oxide Sr3−xSnO},

author = {A Ikeda and T Fukumoto and M Oudah and J N Hausmann and S Yonezawa and S Kobayashi and M Sato and C Tassel and F Takeiri and H Takatsu and H Kageyama and Y Maeno},

url = {http://hdl.handle.net/2433/250419},

doi = {10.1016/j.physb.2017.10.089},

year  = {2018},

date = {2018-05-01},

journal = {Physica B: Condensed Matter},

volume = {536},

pages = {752-756},

abstract = {In order to investigate the position of the strontium deficiency in superconductive Sr3−xSnO, we synthesized and measured X-ray-diffraction patterns of Sr3−xSnO (x∼0.5). Because no clear peaks originating from superstructures were observed, strontium deficiency is most likely to be randomly distributed. We also performed first-principles band-structure calculations on Sr3−xSnO (x=0, 0.5) using two methods: full-potential linearized-augmented plane-wave plus local orbitals method and the Korringa-Kohn-Rostoker Green function method combined with the coherent potential approximation. We revealed that the Fermi energy of Sr3−xSnO in case of x∼0.5 is about 0.8 eV below the original Fermi energy of the stoichiometric Sr3SnO, where the mixing of the valence p and conduction d orbitals are considered to be small. © 2017 Elsevier B.V.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Hausmann2018,

title = {Controlled synthesis of the antiperovskite oxide superconductor Sr3-xSnO},

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

url = {https://arxiv.org/abs/1712.09484},

doi = {10.1088/1361-6668/aab6c2},

year  = {2018},

date = {2018-04-11},

journal = {Superconductor Science and Technology},

volume = {31},

number = {5},

pages = {055012},

abstract = {A large variety of perovskite oxide superconductors are known, including some of the most prominent high-temperature and unconventional superconductors. However, superconductivity among the oxidation state inverted material class, the antiperovskite oxides, was recently reported for the first time. In this superconductor, Sr3-xSnO, the unconventional ionic state Sn4- is realized and possible unconventional superconductivity due to a band inversion has been discussed. Here, we discuss an improved facile synthesis method, making it possible to control the strontium deficiency in Sr3-xSnO. Additionally, a synthesis method above the melting point of Sr3SnO is presented. We show temperature dependence of magnetization and electrical resistivity for superconducting strontium deficient Sr3-xSnO (T c ∼ 5 K) and for Sr3SnO without a superconducting transition in alternating current susceptibility down to 0.15 K. Further, we reveal a significant effect of strontium raw material purity on the superconductivity and achieve substantially increased M/M Meissner (∼1) compared to the highest value reported so far. More detailed characterizations utilizing powder x-ray diffraction and energy-dispersive x-ray spectroscopy show that a minor cubic phase, previously suggested to be another Sr3-xSnO phase with a slightly larger lattice parameter, is SrO. The improved characterization and controlled synthesis reported herein enable detailed investigations on the superconducting nature and its dependency on the strontium deficiency in Sr3-xSnO. © 2018 IOP Publishing Ltd.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Bakurskiy2018,

title = {Local impedance on a rough surface of a chiral p -wave superconductor},

author = {S V Bakurskiy and Ya.V. Fominov and A F Shevchun and Y Asano and Y Tanaka and M.Yu. Kupriyanov and A A Golubov and M R Trunin and H Kashiwaya and S Kashiwaya and Y Maeno},

doi = {10.1103/PhysRevB.98.134508},

year  = {2018},

date = {2018-01-01},

journal = {Physical Review B},

volume = {98},

number = {13},

abstract = {We develop a self-consistent approach for calculating the local impedance at a rough surface of a chiral p-wave superconductor. Using the quasiclassical Eilenberger-Larkin-Ovchinnikov formalism, we numerically find the pair potential, pairing functions, and the surface density of states taking into account diffusive electronic scattering at the surface. The obtained solutions are then employed for studying the local complex conductivity and surface impedance in the broad range of microwave frequencies (ranging from subgap to above-gap values). We identify anomalous features of the surface impedance caused by generation of odd-frequency superconductivity at the surface. The results are compared with experimental data for Sr2RuO4 and provide a microscopic explanation of the phenomenological two-fluid model suggested earlier to explain anomalous features of the microwave response in this material. © 2018 American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Lee2018,

title = {Abnormal phase flip in the coherent phonon oscillations of Ca2RuO4},

author = {M -C Lee and C H Kim and I Kwak and J Kim and S Yoon and B C Park and B Lee and F Nakamura and C Sow and Y Maeno and T W Noh and K W Kim},

doi = {10.1103/PhysRevB.98.161115},

year  = {2018},

date = {2018-01-01},

journal = {Physical Review B},

volume = {98},

number = {16},

abstract = {We employ an optical pump-probe technique to study coherent phonon oscillations in Ca2RuO4. We find that oscillation amplitude of an Ag symmetric phonon mode is strongly suppressed at 260 K, a putative transition point of orbital ordering. The oscillation also shows a gradual but huge change in its phase, such that the oscillation even flips over with a 180 change across the temperature. Density functional theory calculations indicate that the Ag phonon has an eigenmode of octahedral distortion with conventional tilting along the a axis and antipolar distortion of apical oxygen. Careful inspection of the lattice captures an unusually large antipolar distortion in low-temperature structures, which may play a crucial role for the phase transition at 260 K. © 2018 American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{OldeOlthof2018,

title = {Theory of tunneling spectroscopy of normal metal/ferromagnet/spin-triplet superconductor junctions},

author = {L A B Olde Olthof and S -I Suzuki and A A Golubov and M Kunieda and S Yonezawa and Y Maeno and Y Tanaka},

doi = {10.1103/PhysRevB.98.014508},

year  = {2018},

date = {2018-01-01},

journal = {Physical Review B},

volume = {98},

number = {1},

abstract = {We study the tunneling conductance of a ballistic normal metal/ferromagnet/spin-triplet superconductor junction using the extended Blonder-Tinkham-Klapwijk formalism as a model for a c-axis-oriented Au/SrRuO3/Sr2RuO4 junction. We compare chiral p-wave (CPW) and helical p-wave (HPW) pair potentials, combined with ferromagnet magnetization directions parallel and perpendicular to the interface. For fixed θM, where θM is a direction of magnetization in the ferromagnet measured from the c axis, the tunneling conductances of CPW and HPW clearly show different voltage dependencies. It is found that the cases where the d vector is perpendicular to the magnetization direction (CPW with θM=π/2 and HPW with θM=0) are identical. The obtained results serve as a guide to determine the pairing symmetry of the spin-triplet superconductor Sr2RuO4. © 2018 American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Barber2018,

title = {Resistivity in the Vicinity of a van Hove Singularity: Sr2RuO4 under Uniaxial Pressure},

author = {M E Barber and A S Gibbs and Y Maeno and A P Mackenzie and C W Hicks},

doi = {10.1103/PhysRevLett.120.076602},

year  = {2018},

date = {2018-01-01},

journal = {Physical Review Letters},

volume = {120},

number = {7},

abstract = {We report the results of a combined study of the normal-state resistivity and superconducting transition temperature Tc of the unconventional superconductor Sr2RuO4 under uniaxial pressure. There is strong evidence that, as well as driving Tc through a maximum at ∼3.5 K, compressive strains of nearly 1% along the crystallographic [100] axis drive the γ Fermi surface sheet through a van Hove singularity, changing the temperature dependence of the resistivity from T2 above, and below the transition region to T1.5 within it. This occurs in extremely pure single-crystals in which the impurity contribution to the resistivity is <100 nΩ cm, so our study also highlights the potential of uniaxial pressure as a more general probe of this class of physics in clean systems. © 2018 American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Sedeki2018,

title = {Influence of carrier lifetime on quantum criticality and superconducting Tc of (TMTSF) 2ClO4},

author = {A Sedeki and P Auban-Senzier and S Yonezawa and C Bourbonnais and D Jerome},

doi = {10.1103/PhysRevB.98.115111},

year  = {2018},

date = {2018-01-01},

journal = {Physical Review B},

volume = {98},

number = {11},

abstract = {This work presents and analyzes electrical resistivity data on the organic superconductor (TMTSF)2ClO4 and their anion-substituted alloys (TMTSF)2(ClO4)1-x(ReO4)x along the least conducting câ axis. Nonmagnetic disorder introduced by finite size domains of anion ordering on non-Fermi-liquid character of resistivity is investigated near the conditions of quantum criticality taking place at x=0. The evolution of the T-linear resistivity term with anion disorder shows a limited decrease in contrast with the complete suppression of the critical temperature Tc as expected for unconventional superconductivity beyond a threshold value of x. The resulting breakdown of scaling between both quantities is compared to the theoretical predictions of a linearized Boltzmann equation combined to the scaling theory of umklapp scattering in the presence of disorder-induced pair breaking for the carriers. The theory shows that quantum antiferromagnetic fluctuations, which are at the core of unconventional Cooper pairing and inelastic scattering of T-linear resistivity in these systems, are weakly affected by disorder while the phase coherence responsible for a finite Tc can be completely suppressed. © 2018 American Physical Society.},

note = {cited By 0},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Yonezawa2018,

title = {Crossover from impurity-controlled to granular superconductivity in (TMTSF)2 ClO4},

author = {S Yonezawa and C A Marrache-Kikuchi and K Bechgaard and D Jérome},

doi = {10.1103/PhysRevB.97.014521},

year  = {2018},

date = {2018-01-01},

journal = {Physical Review B},

volume = {97},

number = {1},

abstract = {Using a proper cooling procedure, a controllable amount of nonmagnetic structural disorder can be introduced at low temperature in (TMTSF)2ClO4. Here we performed simultaneous measurements of transport and magnetic properties of (TMTSF)2ClO4 in its normal and superconducting states, while finely covering three orders of magnitude of the cooling rate around the anion ordering temperature. Our result reveals, with increasing density of disorder, the existence of a crossover between homogeneous defect-controlled d-wave superconductivity and granular superconductivity. At slow cooling rates, with small amount of disorder, the evolution of superconducting properties is well described with the Abrikosov-Gorkov theory, providing further confirmation of non-s-wave pairing in this compound. In contrast, at fast cooling rates, zero resistance and diamagnetic shielding are achieved through a randomly distributed network of superconducting puddles embedded in a normal conducting background and interconnected by proximity effect coupling. The temperature dependence of the ac complex susceptibility reveals features typical for a network of granular superconductors. This makes (TMTSF)2ClO4 a model system for granular superconductivity where the grain size and their concentration are tunable within the same sample. © 2018 American Physical Society.},

note = {cited By 5},

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}

}