Shunsaku KITAGAWA

@article{Okuno_JPSJ_2020,

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

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

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

doi = {10.7566/JPSJ.89.095002},

year  = {2020},

date = {2020-08-03},

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

volume = {89},

number = {9},

pages = {095002},

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

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{S_Kitagawa2020,

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

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

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

doi = {10.7566/JPSJ.89.053701},

year  = {2020},

date = {2020-04-02},

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

volume = {89},

number = {5},

pages = {053701},

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

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Okuno2020,

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

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

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

doi = {10.1103/PhysRevB.101.121406},

year  = {2020},

date = {2020-03-30},

journal = {Physical Review B},

volume = {101},

number = {12},

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

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Manago2019c,

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

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

doi = {10.7566/JPSJ.88.113704},

year  = {2019},

date = {2019-10-23},

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

volume = {88},

number = {11},

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

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Nakamine2019b,

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

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

doi = {10.7566/JPSJ.88.113703},

year  = {2019},

date = {2019-10-17},

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

volume = {88},

number = {11},

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

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kitagawa2019,

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

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

doi = {10.1103/PhysRevB.100.060503},

year  = {2019},

date = {2019-08-12},

journal = {Physical Review B},

volume = {100},

number = {6},

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

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Asano2019,

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

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

doi = {10.7566/JPSJ.88.084709},

year  = {2019},

date = {2019-07-23},

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

volume = {88},

number = {8},

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

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Manago2019,

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

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

doi = {10.1103/PhysRevB.100.035203},

year  = {2019},

date = {2019-07-12},

journal = {Physical Review B},

volume = {100},

number = {3},

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

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Tokunaga2019,

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

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

doi = {10.7566/JPSJ.88.073701},

year  = {2019},

date = {2019-06-24},

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

volume = {88},

number = {7},

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

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kinjo2019,

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

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

doi = {10.7566/JPSJ.88.065002},

year  = {2019},

date = {2019-05-30},

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

volume = {88},

number = {6},

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

keywords = {},

pubstate = {published},

tppubtype = {article}

}