Publication list

@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}

}

@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{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{Yasui2017,

title = {Little-Parks oscillations with half-quantum fluxoid features in Sr_{2}RuO_{4} microrings},

author = {Y Yasui and K Lahabi and M S Anwar and Y Nakamura and S Yonezawa and T Terashima and J Aarts and Y Maeno},

doi = {10.1103/PhysRevB.96.180507},

year  = {2017},

date = {2017-11-27},

journal = {Physical Review B},

volume = {96},

number = {18},

abstract = {In a microring of a superconductor with a spin-triplet equal-spin pairing state, a fluxoid, a combined object of magnetic flux and circulating supercurrent, can penetrate as half-integer multiples of the flux quantum. A candidate material to investigate such half-quantum fluxoids is Sr 2 RuO 4 . We fabricated Sr 2 RuO 4 microrings using single crystals and measured their resistance behavior under magnetic fields controlled with a three-axis vector magnet. Proper Little-Parks oscillations in the magnetovoltage as a function of an axially applied field, associated with fluxoid quantization, are clearly observed using bulk single-crystalline superconductors. We then performed magnetovoltage measurements with additional in-plane magnetic fields. By carefully analyzing both the voltages V+ (V-) measured at positive (negative) current, we find that, above an in-plane threshold field of about 10 mT, the magnetovoltage maxima convert to minima. We interpret this behavior as the peak splitting expected for the half-quantum fluxoid states. © 2017 American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Mackenzie2017,

title = {Even odder after twenty-three years: The superconducting order parameter puzzle of Sr2RuO4},

author = {A P Mackenzie and T Scaffidi and C W Hicks and Y Maeno},

doi = {10.1038/s41535-017-0045-4},

year  = {2017},

date = {2017-01-01},

journal = {npj Quantum Materials},

volume = {2},

number = {1},

abstract = {In this short review, we aim to provide a topical update on the status of efforts to understand the superconductivity of Sr2RuO4. We concentrate on efforts to identify a superconducting order parameter symmetry that is compatible with all the major pieces of experimental knowledge of the material, and highlight some major discrepancies that have become even clearer in recent years. As the pun in the title suggests, we have tried to start the discussion from scratch, making no assumptions even about fundamental issues such as the parity of the superconducting state. We conclude that no consensus is currently achievable in Sr2RuO4, and that the reasons for this go to the heart of how well some of the key probes of unconventional superconductivity are really understood. This is, therefore, a puzzle that merits continued in-depth study. © 2017 The Author(s).},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Sow20171084,

title = {Current-induced strong diamagnetism in the Mott insulator Ca2RuO4},

author = {C Sow and S Yonezawa and S Kitamura and T Oka and K Kuroki and F Nakamura and Y Maeno},

doi = {10.1126/science.aah4297},

year  = {2017},

date = {2017-01-01},

journal = {Science},

volume = {358},

number = {6366},

pages = {1084-1087},

abstract = {Mott insulators can host a surprisingly diverse set of quantum phenomena when their frozen electrons are perturbed by various stimuli. Superconductivity, metal-insulator transition, and colossal magnetoresistance induced by element substitution, pressure, and magnetic field are prominent examples. Here we report strong diamagnetism in the Mott insulator calcium ruthenate (Ca2RuO4) induced by dc electric current. The application of a current density of merely 1 ampere per centimeter squared induces diamagnetism stronger than that in other nonsuperconducting materials. This change is coincident with changes in the transport properties as the system becomes semimetallic. These findings suggest that dc current may be a means to control the properties of materials in the vicinity of a Mott insulating transition. © 2017, American Association for the Advancement of Science. All rights reserved.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Sakuma2017,

title = {Investigation of the vortex states of Sr2RuO4-Ru eutectic microplates using DC-SQUIDs},

author = {D Sakuma and Y Nago and R Ishiguro and S Kashiwaya and S Nomura and K Kono and Y Maeno and H Takayanagi},

doi = {10.7566/JPSJ.86.114708},

year  = {2017},

date = {2017-01-01},

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

volume = {86},

number = {11},

abstract = {We investigated the magnetic properties of a Sr2RuO4-Ru eutectic microplate containing a single Ru-inclusion using micrometer-sized DC-SQUIDs (direct-current superconducting quantum interference devices). A phase frustration at the interface between chiral p-wave superconducting Sr2RuO4 and s-wave superconducting Ru is expected to cause novel magnetic vortex states such as the spontaneous Ru-center vortex under zero magnetic field [as reported by H. Kaneyasu and M. Sigrist, J. Phys. Soc. Jpn. 79, 053706 (2010)]. Our experimental results show no positive evidence for such a spontaneous vortex state. However, in an applied field, an abrupt change in the magnetic flux distribution was observed at a superconducting transition of Ru. The flux distribution is clarified by comparing our experimental results with electromagnetic field simulations in our sample geometry. We discuss the transition of the vortex states and the superconducting coupling at the Sr2RuO4=Ru interface © 2017 The Physical Society of Japan.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kuhn2017,

title = {Anisotropy and multiband superconductivity in Sr2RuO4 determined by small-angle neutron scattering studies of the vortex lattice},

author = {S J Kuhn and W Morgenlander and E R Louden and C Rastovski and W J Gannon and H Takatsu and D C Peets and Y Maeno and C D Dewhurst and J Gavilano and M R Eskildsen},

doi = {10.1103/PhysRevB.96.174507},

year  = {2017},

date = {2017-01-01},

journal = {Physical Review B},

volume = {96},

number = {17},

abstract = {Despite numerous studies the exact nature of the order parameter in superconducting Sr2RuO4 remains unresolved. We have extended previous small-angle neutron scattering studies of the vortex lattice in this material to a wider field range, higher temperatures, and with the field applied close to both the (100) and (110) basal plane directions. Measurements at high field were made possible by the use of both spin polarization and analysis to improve the signal-to-noise ratio. Rotating the field towards the basal plane causes a distortion of the square vortex lattice observed for H(001) and also a symmetry change to a distorted triangular symmetry for fields close to (100).The vortex lattice distortion allows us to determine the intrinsic superconducting anisotropy between the c axis and the Ru-O basal plane, yielding a value of ∼60 at low temperature and low to intermediate fields. This greatly exceeds the upper critical field anisotropy of ∼20 at low temperature, reminiscent of Pauli limiting. Indirect evidence for Pauli paramagnetic effects on the unpaired quasiparticles in the vortex cores are observed, but a direct detection lies below the measurement sensitivity. The superconducting anisotropy is found to be independent of temperature but increases for fields 1 T, indicating multiband superconductvity in Sr2RuO4. Finally, the temperature dependence of the scattered intensity provides further support for gap nodes or deep minima in the superconducting gap. © 2017 American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Wang2017799,

title = {Quasiparticle interference and strong electron-mode coupling in the quasi-one-dimensional bands of Sr 2 RuO 4},

author = {Z Wang and D Walkup and P Derry and T Scaffdi and M Rak and S Vig and A Kogar and I Zeljkovic and A Husain and L H Santos and Y Wang and A Damascelli and Y Maeno and P Abbamonte and E Fradkin and V Madhavan},

doi = {10.1038/NPHYS4107},

year  = {2017},

date = {2017-01-01},

journal = {Nature Physics},

volume = {13},

number = {8},

pages = {799-805},

abstract = {The single-layered ruthenate Sr 2 RuO 4 is presented as a potential spin-triplet superconductor with an order parameter that may break time-reversal invariance and host half-quantized vortices with Majorana zero modes. Although the actual nature of the superconducting state is still a matter of controversy, it is believed to condense from a metallic state that is well described by a conventional Fermi liquid. In this work we use a combination of Fourier transform scanning tunnelling spectroscopy (FT-STS) and momentum-resolved electron energy loss spectroscopy (M-EELS) to probe interaction effects in the normal state of Sr 2 RuO 4 . Our high-resolution FT-STS data show signatures of the β-band with a distinctly quasi-one-dimensional (1D) character. The band dispersion reveals surprisingly strong interaction effects that dramatically renormalize the Fermi velocity, suggesting that the normal state of Sr 2 RuO 4 is that of a 'correlated metal' where correlations are strengthened by the quasi-1D nature of the bands. In addition, kinks at energies of approximately 10 meV, 38 meV and 70 meV are observed. By comparing STM and M-EELS data we show that the two higher energy features arise from coupling with collective modes. The strong correlation effects and the kinks in the quasi-1D bands could provide important information for understanding the superconducting state. © 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Anwar2017,

title = {Multicomponent order parameter superconductivity of Sr2RuO4 revealed by topological junctions},

author = {M S Anwar and R Ishiguro and T Nakamura and M Yakabe and S Yonezawa and H Takayanagi and Y Maeno},

doi = {10.1103/PhysRevB.95.224509},

year  = {2017},

date = {2017-01-01},

journal = {Physical Review B},

volume = {95},

number = {22},

abstract = {Single crystals of the Sr2RuO4-Ru eutectic system are known to exhibit enhanced superconductivity at 3 K in addition to the bulk superconductivity of Sr2RuO4 at 1.5 K. The 1.5 K phase is believed to be a spin-triplet, chiral p-wave state with a multicomponent order parameter, giving rise to chiral domain structure. In contrast, the 3 K phase is attributable to enhanced superconductivity of Sr2RuO4 in the strained interface region between Ru inclusion of a few to tens of micrometers in size and the surrounding Sr2RuO4. We investigate the dynamic behavior of a topological junction, where a superconductor is surrounded by another superconductor. Specifically, we fabricated Nb/Ru/Sr2RuO4 topological superconducting junctions, in which the difference in phase winding between the s-wave superconductivity in Ru microislands induced from Nb and the superconductivity of Sr2RuO4 mainly governs the junction behavior. Comparative results of the asymmetry, hysteresis, and noise in junctions with different sizes, shapes, and configurations of Ru inclusions are explained by the chiral domain-wall motion in these topological junctions. Furthermore, a striking difference between the 1.5 and 3 K phases is clearly revealed: the large noise in the 1.5 K phase sharply disappears in the 3 K phase. These results confirm the multicomponent order-parameter superconductivity of the bulk Sr2RuO4, consistent with the chiral p-wave state, and the proposed nonchiral single-component superconductivity of the 3 K phase. © 2017 American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kunkemöller2017,

title = {Absence of a Large Superconductivity-Induced Gap in Magnetic Fluctuations of Sr2RuO4},

author = {S Kunkemöller and P Steffens and P Link and Y Sidis and Z Q Mao and Y Maeno and M Braden},

doi = {10.1103/PhysRevLett.118.147002},

year  = {2017},

date = {2017-01-01},

journal = {Physical Review Letters},

volume = {118},

number = {14},

abstract = {Inelastic neutron scattering experiments on Sr2RuO4 determine the spectral weight of the nesting induced magnetic fluctuations across the superconducting transition. There is no observable change at the superconducting transition down to an energy of ∼0.35 meV, which is well below the 2Δ values reported in several tunneling experiments. At this and higher energies magnetic fluctuations clearly persist in the superconducting state. Only at energies below ∼0.3 meV can evidence for partial suppression of spectral weight in the superconducting state be observed. This strongly suggests that the one-dimensional bands with the associated nesting fluctuations do not form the active, highly gapped bands in the superconducting pairing in Sr2RuO4. © 2017 American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Hassinger2017,

title = {Vertical line nodes in the superconducting gap structure of Sr2RuO4},

author = {E Hassinger and P Bourgeois-Hope and H Taniguchi and S René de Cotret and G Grissonnanche and M S. Anwar and Y Maeno and N Doiron-Leyraud and L Taillefer},

doi = {10.1103/PhysRevX.7.011032},

year  = {2017},

date = {2017-01-01},

journal = {Physical Review X},

volume = {7},

number = {1},

abstract = {There is strong experimental evidence that the superconductor Sr2RuO4 has a chiral p-wave order parameter. This symmetry does not require that the associated gap has nodes, yet specific heat, ultrasound, and thermal conductivity measurements indicate the presence of nodes in the superconducting gap structure of Sr2RuO4. Theoretical scenarios have been proposed to account for the existence of deep minima or accidental nodes (minima tuned to zero or below by material parameters) within a p-wave state. Other scenarios propose chiral d-wave and f-wave states, with horizontal and vertical line nodes, respectively. To elucidate the nodal structure of the gap, it is essential to know whether the lines of nodes (or minima) are vertical (parallel to the tetragonal c axis) or horizontal (perpendicular to the c axis). Here, we report thermal conductivity measurements on single crystals of Sr2RuO4 down to 50 mK for currents parallel and perpendicular to the c axis. We find that there is substantial quasiparticle transport in the T = 0 limit for both current directions. A magnetic field H immediately excites quasiparticles with velocities both in the basal plane and in the c direction. Our data down to Tc/30 and down to Hc2=100 show no evidence that the nodes are in fact deep minima. Relative to the normal state, the thermal conductivity of the superconducting state is found to be very similar for the two current directions, from H = 0 to H = Hc2. These findings show that the gap structure of Sr2RuO4 consists of vertical line nodes. This rules out a chiral d-wave state. Given that the c-axis dispersion (warping) of the Fermi surface in Sr2RuO4 varies strongly from sheet to sheet, the small a - c anisotropy suggests that the line nodes are present on all three sheets of the Fermi surface. If imposed by symmetry, vertical line nodes would be inconsistent with a p-wave order parameter for Sr2RuO4. To reconcile the gap structure revealed by our data with a p-wave state, a mechanism must be found that produces accidental line nodes in Sr2RuO4.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Yonezawa2017123,

title = {Thermodynamic evidence for nematic superconductivity in Cu x Bi 2 Se 3},

author = {S Yonezawa and K Tajiri and S Nakata and Y Nagai and Z Wang and K Segawa and Y Ando and Y Maeno},

doi = {10.1038/nphys3907},

year  = {2017},

date = {2017-01-01},

journal = {Nature Physics},

volume = {13},

number = {2},

pages = {123-126},

abstract = {In condensed matter physics, spontaneous symmetry breaking has been a key concept, and discoveries of new types of broken symmetries have greatly increased our understanding of matter 1,2 . Recently, electronic nematicity, novel spontaneous rotational-symmetry breaking leading to an emergence of a special direction in electron liquids, has been attracting significant attention 3-6 . Here, we show bulk thermodynamic evidence for nematic superconductivity, in which the nematicity emerges in the superconducting gap amplitude, in Cu x Bi 2 Se 3 . Based on high-resolution calorimetry of single-crystalline samples under accurate two-axis control of the magnetic field direction, we discovered clear two-fold symmetry in the specific heat and in the upper critical field despite the trigonal symmetry of the lattice. Nematic superconductivity for this material should possess a unique topological nature associated with odd parity 7-9 . Thus, our findings establish a new class of spontaneously symmetry-broken states of matter-namely, odd-parity nematic superconductivity. © 2017 Macmillan Publishers Limited.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}