石田 憲二

@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.},

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pubstate = {published},

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}

@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.},

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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.},

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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.},

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pubstate = {published},

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}

@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.},

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pubstate = {published},

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}

@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.},

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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.},

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pubstate = {published},

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}

@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.},

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pubstate = {published},

tppubtype = {article}

}

@article{Kitagawa2017,

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

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

doi = {10.1103/PhysRevB.96.134506},

year  = {2017},

date = {2017-01-01},

journal = {Physical Review B},

volume = {96},

number = {13},

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

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pubstate = {published},

tppubtype = {article}

}

@article{Yamanaka2017,

title = {Magnetic and superconducting properties of a heavy-fermion CeCoIn5 epitaxial film probed by nuclear quadrupole resonance},

author = {T Yamanaka and M Shimozawa and H Shishido and S Kitagawa and H Ikeda and T Shibauchi and T Terashima and Y Matsuda and K Ishida},

doi = {10.1103/PhysRevB.96.060507},

year  = {2017},

date = {2017-01-01},

journal = {Physical Review B},

volume = {96},

number = {6},

abstract = {Since the progress in the fabrication techniques of thin films of exotic materials such as strongly correlated heavy-fermion compounds, microscopic studies of the magnetic and electronic properties inside the films have been needed. Herein, we report the observation of In115 nuclear quadrupole resonance (NQR) in an epitaxial film of the heavy-fermion superconductor CeCoIn5, for which the microscopic field gradient within the unit cell as well as magnetic and superconducting properties at zero field are evaluated. We find that the nuclear spin-lattice relaxation rate in the film is in excellent agreement with that of bulk crystals, whereas the NQR spectra show noticeable shifts and significant broadening indicating a change in the electric-field distribution inside the film. The analysis implies a displacement of In layers in the film, which, however, does not affect the magnetic fluctuations and superconducting pairing. This implies that inhomogeneity of the electronic field gradient in the film sample causes no pair-breaking effect. © 2017 American Physical Society.},

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pubstate = {published},

tppubtype = {article}

}

@article{Manago2017,

title = {Absence of the Pauli-paramagnetic limit in a superconducting U6Co},

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

doi = {10.7566/JPSJ.86.073701},

year  = {2017},

date = {2017-01-01},

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

volume = {86},

number = {7},

abstract = {We performed 59Co nuclear magnetic resonance (NMR) measurements of single-crystalline U6Co. There is a small decrease in the Knight shift in the superconducting (SC) state, but this change mainly arises from the SC diamagnetic effect. The negligible change of the spin part of the Knight shift, together with the absence of the Pauli-paramagnetic effect in the SC U6Co, is understood as a consequence of the small spin susceptibility. The nuclear spin–lattice relaxation rate 1/T1 is also measured in the SC state under the magnetic field, and exhibits a tiny Hebel–Slichter peak just below the SC transition temperature and exponential behavior at lower temperatures. These behaviors are in agreement with the full-gap s-wave pairing in U6Co. ©2017 The Physical Society of Japan.},

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tppubtype = {article}

}

@article{Manago2016,

title = {Anomalous magnetic fluctuations in superconducting Sr2RuO4 revealed by Ru 101 nuclear spin-spin relaxation},

author = {M Manago and T Yamanaka and K Ishida and Z Mao and Y Maeno},

doi = {10.1103/PhysRevB.94.144511},

year  = {2016},

date = {2016-01-01},

journal = {Physical Review B},

volume = {94},

number = {14},

abstract = {We carried out Ru101 nuclear quadrupole resonance (NQR) measurement on superconducting (SC) Sr 2 RuO 4 under zero magnetic field (H=0) and found that the nuclear spin-spin relaxation rate 1/T 2 is enhanced in the SC state. The 1/T 2 measurement in the SC state under H=0 is effective for detecting slow magnetic fluctuations parallel to the quantized axis of the nuclear spin. Our results indicate that low-energy magnetic fluctuations perpendicular to the RuO 2 plane emerge when the superconductivity sets in, which is consistent with the previous O17-NQR result that the nuclear spin-lattice relaxation rate 1/T1 of the in-plane O site exhibits anomalous behavior in the SC state. The enhancement of the magnetic fluctuations in the SC state is unusual and suggests that the fluctuations are related to the unconventional SC pairing. We suggest that this phenomenon is a consequence of the spin degrees of freedom of the spin-triplet pairing. © 2016 American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Manago2016b,

title = {Absence of the O 17 Knight-shift changes across the first-order phase transition line in Sr2RuO4},

author = {M Manago and K Ishida and Z Mao and Y Maeno},

doi = {10.1103/PhysRevB.94.180507},

year  = {2016},

date = {2016-01-01},

journal = {Physical Review B},

volume = {94},

number = {18},

abstract = {We performed O17 nuclear magnetic resonance measurements on superconducting (SC) Sr2RuO4 under in-plane magnetic fields. We found that no new signal appears in the SC state and that the O17 Knight shifts obtained from the double-site measurements remain constant across the first-order phase-transition line, as well as across the second-order phase-transition line as already reported. The present results indicate that the SC spin susceptibility does not decrease in the high-field region, although a magnetization jump in the SC state was reported at low temperatures. Because the spin susceptibility is unchanged in the SC state in Sr2RuO4, we suggest that the first-order phase transition across the upper critical field should be interpreted as a depairing mechanism other than the conventional Pauli-paramagnetic effect. © 2016 American Physical Society.},

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pubstate = {published},

tppubtype = {article}

}

@article{Kasahara2016,

title = {Giant superconducting fluctuations in the compensated semimetal FeSe at the BCS-BEC crossover},

author = {S Kasahara and T Yamashita and A Shi and R Kobayashi and Y Shimoyama and T Watashige and K Ishida and T Terashima and T Wolf and F Hardy and C Meingast and H V Löhneysen and A Levchenko and T Shibauchi and Y Matsuda},

doi = {10.1038/ncomms12843},

year  = {2016},

date = {2016-01-01},

journal = {Nature Communications},

volume = {7},

abstract = {The physics of the crossover between weak-coupling Bardeen-Cooper-Schrieffer (BCS) and strong-coupling Bose-Einstein condensate (BEC) limits gives a unified framework of quantum-bound (superfluid) states of interacting fermions. This crossover has been studied in the ultracold atomic systems, but is extremely difficult to be realized for electrons in solids. Recently, the superconducting semimetal FeSe with a transition temperature Tc =8.5 K has been found to be deep inside the BCS-BEC crossover regime. Here we report experimental signatures of preformed Cooper pairing in FeSe, whose energy scale is comparable to the Fermi energies. In stark contrast to usual superconductors, large non-linear diamagnetism by far exceeding the standard Gaussian superconducting fluctuations is observed below T∗∼20 K, providing thermodynamic evidence for prevailing phase fluctuations of superconductivity. Nuclear magnetic resonance and transport data give evidence of pseudogap formation at ∼T∗. The multiband superconductivity along with electron-hole compensation in FeSe may highlight a novel aspect of the BCS-BEC crossover physics. © The Author(s) 2016.},

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pubstate = {published},

tppubtype = {article}

}

@article{Ishida2015,

title = {Spin polarization enhanced by spin-triplet pairing in Sr2RuO4 probed by NMR},

author = {K Ishida and M Manago and T Yamanaka and H Fukazawa and Z Q Mao and Y Maeno and K Miyake},

doi = {10.1103/PhysRevB.92.100502},

year  = {2015},

date = {2015-01-01},

journal = {Physical Review B - Condensed Matter and Materials Physics},

volume = {92},

number = {10},

abstract = {We report a phenomenon intimately related to the spin-triplet superconductivity. It is well known that the spin susceptibility decreases below the superconducting transition temperature in almost all superconductors because of spin-singlet pair formation, while it may remain unchanged in a handful of spin-triplet exceptions. Here we report the observation in Sr2RuO4 with nuclear magnetic resonance that the spin susceptibility originating from the Ru-4d electron slightly increases by ∼2% of the total and becomes inhomogeneous in the superconducting state. These are reasonably explained if the electron pairs form the equal-spin pairing in the mixed state. A similar phenomenon was predicted for superfluid He340 years ago, but had never been demonstrated in any superconductor. © 2015 American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Yamanaka2015,

title = {Interface between heavy fermions and normal electrons investigated by spatially resolved nuclear magnetic resonance},

author = {T Yamanaka and M Shimozawa and R Endo and Y Mizukami and H Shishido and T Terashima and T Shibauchi and Y Matsuda and K Ishida},

doi = {10.1103/PhysRevB.92.241105},

year  = {2015},

date = {2015-01-01},

journal = {Physical Review B - Condensed Matter and Materials Physics},

volume = {92},

number = {24},

abstract = {We have studied the superlattices with alternating block layers (BLs) of heavy-fermion superconductor CeCoIn5 and conventional-metal YbCoIn5 by site-selective nuclear magnetic resonance spectroscopy, which uniquely offers spatially resolved dynamical magnetic information. We find that the presence of antiferromagnetic fluctuations is confined to the Ce BLs, indicating that magnetic degrees of freedom of felectrons are quenched inside the Yb BLs. Contrary to simple expectations that the two dimensionalization enhances fluctuations, we observe that antiferromagnetic fluctuations are rapidly suppressed with decreasing Ce BL thickness. Moreover, the suppression is more prominent near the interfaces between the BLs. These results imply significant effects of local inversion symmetry breaking at the interfaces. © 2015 American Physical Society.},

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pubstate = {published},

tppubtype = {article}

}

@article{Manago2015,

title = {Effect of Geomagnetism on 101Ru Nuclear Quadrupole Resonance Measurements of CeRu2},

author = {M Manago and K Ishida and T D Matsuda and Y Onuki},

doi = {10.7566/JPSJ.84.115001},

year  = {2015},

date = {2015-01-01},

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

volume = {84},

number = {11},

abstract = {We performed 101Ru nuclear quadrupole resonance (NQR) measurements on the s-wave superconductor CeRu2 and found oscillatory behavior in the spin-echo amplitude at the |±1/2 虠 |±3/2 transitions but not at the |±3/2 虠 |±5/2 transitions. The modulation disappears in the superconducting state or in a magnetic shield, which implies a geomagnetic field effect. Our results indicate that the NQR spin-echo decay curve at the |±1/2 虠 |±3/2 transitions is sensitive to a weak magnetic field. © 2015 The Physical Society of Japan.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kitagawa2015,

title = {Suppression of nonmagnetic insulating state by application of pressure in mineral tetrahedrite Cu12Sb4S13},

author = {S Kitagawa and T Sekiya and S Araki and T C Kobayashi and K Ishida and T Kambe and T Kimura and N Nishimoto and K Kudo and M Nohara},

doi = {10.7566/JPSJ.84.093701},

year  = {2015},

date = {2015-01-01},

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

volume = {84},

number = {9},

abstract = {The mineral tetrahedrite Cu12Sb4S13 exhibits a first-order metal-insulator transition (MIT) at TMI = 85K and ambient pressure. We measured the 63Cu-NMR at ambient pressure and the resistivity and magnetic susceptibility at high pressures. 63Cu-NMR results indicate a nonmagnetic insulating ground state in this compound. The MIT is monotonically suppressed by pressure and disappears at ~1.0 GPa. Two other anomalies are observed in the resistivity measurements, and the pressure-temperature phase diagram of Cu12Sb4S13 is constructed. ©2015 The Physical Society of Japan.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Iye2015,

title = {Emergence of orbital nematicity in the tetragonal phase of BaFe2(As1-xPx)2},

author = {T Iye and M -H Julien and H Mayaffre and M Horvatić and C Berthier and K Ishida and H Ikeda and S Kasahara and T Shibauchi and Y Matsuda},

doi = {10.7566/JPSJ.84.043705},

year  = {2015},

date = {2015-01-01},

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

volume = {84},

number = {4},

abstract = {We report on 75As-NMR measurements in single crystalline BaFe2(As0.96P0.04)2 for magnetic fields parallel to the orthorhombic [110]o and [100]o directions above the structural transition temperature TS ≃ 121 K. A large difference in the linewidth between the two field directions reveals in-plane anisotropy of the electric field gradient, even in the tetragonal phase. This provides microscopic evidence of population imbalance between As-4px and 4py orbitals, which reaches |nx - ny| /|nx + ny| ∼ 15% at T → TS and is a natural consequence of the orbital ordering of Fe-3dxz and dyz electrons. Surprisingly, this orbital polarization is found to be already static near room temperature, suggesting that it arises from the pinning of anisotropic orbital fluctuations by disorder. The effect is found to be stronger below ∼160 K, which coincides with the appearance of nematicity in previous torque and photoemission measurements. These results impose strong constraints on microscopic models of the nematic state. ©2015 The Physical Society of Japan.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Karube2015,

title = {Ferromagnetic critical behavior in U(Co1-xFex)Al (0≤x≤0.02) studied by Co 59 nuclear quadrupole resonance measurements FERROMAGNETIC CRITICAL BEHAVIOR in U(Co ... K. KARUBE, T. HATTORI, K. ISHIDA, and N. KIMURA},

author = {K Karube and T Hattori and K Ishida and N Kimura},

doi = {10.1103/PhysRevB.91.075131},

year  = {2015},

date = {2015-01-01},

journal = {Physical Review B - Condensed Matter and Materials Physics},

volume = {91},

number = {7},

abstract = {In order to investigate physical properties around a ferromagnetic (FM) quantum transition point and a tricritical point (TCP) in the itinerant-electron metamagnetic compound UCoAl, we have performed the Co59 nuclear quadrupole resonance (NQR) measurement for the Fe-substituted U(Co1-xFex)Al(x=0,0.5,1,and2%) in zero external magnetic field. The Fe concentration dependence of Co59-NQR spectra at low temperatures indicates that the first-order FM transition occurs at least above x=1%. The magnetic fluctuations along the c axis detected by the nuclear spin-spin relaxation rate 1/T2 exhibit an anomaly at Tmax∼20K and enhance with increasing x. These results are in good agreement with theoretical predictions and indicate the presence of prominent critical fluctuations at the TCP in this system. © 2015 American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Böhmer2015,

title = {Origin of the tetragonal-to-orthorhombic phase transition in FeSe: A combined thermodynamic and NMR study of nematicity},

author = {A E Böhmer and T Arai and F Hardy and T Hattori and T Iye and T Wolf and H V Löhneysen and K Ishida and C Meingast},

doi = {10.1103/PhysRevLett.114.027001},

year  = {2015},

date = {2015-01-01},

journal = {Physical Review Letters},

volume = {114},

number = {2},

abstract = {The nature of the tetragonal-to-orthorhombic structural transition at Ts≈90K in single crystalline FeSe is studied using shear-modulus, heat-capacity, magnetization, and nuclear magnetic resonance measurements. The transition is shown to be accompanied by a large shear-modulus softening, which is practically identical to that of underdoped Ba(Fe,Co)2As2, suggesting a very similar strength of the electron-lattice coupling. On the other hand, a spin-fluctuation contribution to the spin-lattice relaxation rate is only observed below Ts. This indicates that the structural, or "nematic," phase transition in FeSe is not driven by magnetic fluctuations. © 2015 American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Karube2014,

title = {Anisotropic uniaxial pressure response in UCoAl studied by nuclear magnetic resonance measurement},

author = {K Karube and S Kitagawa and T Hattori and K Ishida and N Kimura and T Komatsubara},

doi = {10.7566/JPSJ.83.084706},

year  = {2014},

date = {2014-01-01},

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

volume = {83},

number = {8},

abstract = {We have performed nuclear quadrupole resonance and nuclear magnetic resonance measurements on UCoAl with strong Ising-type anisotropy under b- and c-axes uniaxial pressure. In the b-axis uniaxial pressure ( P∥b) measurement, we observed an increase in the metamagnetic transition field with increasing P∥b. In the c-axis uniaxial pressure (P∥b) measurement, on the other hand, we observed a ferromagnetic transition in zero magnetic field along the c-axis above P∥c= 0.08 GPa. The anomaly of the nuclear spin-lattice relaxation rate divided by the temperature [(T1T)-1] at T = 20 K is suppressed by P∥band slightly enhanced by P∥c. The anisotropic uniaxial pressure response indicates that uniaxial pressure is a good parameter for tuning the Ising magnetism in UCoAl. © 2014 The Physical Society of Japan.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Hattori2014,

title = {Relationship between ferromagnetic criticality and the enhancement of superconductivity induced by transverse magnetic fields in UCoGe},

author = {T Hattori and K Karube and K Ishida and K Deguchi and N K Sato and T Yamamura},

doi = {10.7566/JPSJ.83.073708},

year  = {2014},

date = {2014-01-01},

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

volume = {83},

number = {7},

abstract = {We have performed59Co NMR experiments on the ferromagnetic superconductor UCoGe under magnetic fields (H) along the a- and b-axes to investigate the relationship between ferromagnetic properties and superconductivity. The ferromagnetic ordering temperature TCurie is suppressed and the nuclear spin-lattice relaxation rate 1/T1at 2K is enhanced in H ∥ b, although TCurie and 1/T1are unchanged in H ∥ a, indicating that the ferromagnetic criticality is induced only when H is applied along the b-axis. We show the close relationship between the magnetic anisotropies and the superconducting ones reported by Aoki et al.: the superconductivity is gradually suppressed in H ∥ a, but enhanced in H ∥ b above 5 T. We strongly suggest that the enhancement of the superconductivity observed in H ∥ b originates from the field induced ferromagnetic criticality, as pointed out by Aoki et al. and Mineev. © 2014 The Physical Society of Japan.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Hattori2014b,

title = {Spin-triplet superconductivity induced by longitudinal ferromagnetic fluctuations in UCoGe probed by59Co NMR measurement},

author = {T Hattori and Y Ihara and K Karube and D Sugimoto and K Ishida and K Deguchi and N K Sato and T Yamamura},

doi = {10.7566/JPSJ.83.061012},

year  = {2014},

date = {2014-01-01},

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

volume = {83},

number = {6},

abstract = {In this review, we present a summary of our experimental results suggesting that the superconducting ferromagnet UCoGe is a spin-triplet superconductor, along with some of the basic physical properties of UCoGe in the normal state. We show experimental evidence that superconductivity in UCoGe is mediated by longitudinal ferromagnetic fluctuations predominant above the superconducting-transition temperature TSC. This is obtained from the tuning of ferromagnetic fluctuations and superconductivity by changing the external field along the magnetic easy axis, the c-axis. In addition, we show that the spin susceptibility perpendicular to the spontaneous magnetic moment, which is probed by Knight-shift measurement, is almost constant in the superconducting state. The behavior of the susceptibility and the invariance of the spontaneous moment below TSCcan be consistently explained by the equal-spin pairing state with a large exchange field along the c-axis. © 2014 The Physical Society of Japan.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kitagawa2014,

title = {Relationship between superconductivity and antiferromagnetism in LaFe(As1-xPx)O revealed by 31P-NMR},

author = {S Kitagawa and T Iye and Y Nakai and K Ishida and C Wang and G -H Cao and Z -A Xu},

doi = {10.7566/JPSJ.83.023707},

year  = {2014},

date = {2014-01-01},

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

volume = {83},

number = {2},

abstract = {We performed 31P-NMR measurements on LaFe(As1-xP x)O to investigate the relationship between antiferromagnetism and superconductivity. The antiferromagnetic (AFM) ordering temperature T N and the moment μord are continuously suppressed with increasing P content x and disappear at x = 0.3 where bulk superconductivity appears. At this superconducting x = 0.3, quantum critical AFM fluctuations are observed, indicative of the intimate relationship between superconductivity and low-energy AFM fluctuations associated with the quantum-critical point in LaFe(As 1-xPx)O. The relationship is similar to those observed in other isovalent-substitution systems, e.g., BaFe2(As 1-xPx)2 and SrFe2(As 1-xPx)2, with the "122" structure. Moreover, the AFM order reappears with further P substitution (x 0.4). The variation of the ground state with respect to the P substitution is considered to be linked to the change in the band character of Fe-3d orbitals around the Fermi level. © 2014 The Physical Society of Japan.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Mito2013,

title = {Microscopic evidence of a crossover to a low-temperature intermediate valence state in YbCo2Zn20},

author = {T Mito and H Hara and T Ishida and K Nakagawara and T Koyama and K Ueda and T Kohara and K Ishida and K Matsubayashi and Y Saiga and Y Uwatoko},

doi = {10.7566/JPSJ.82.103704},

year  = {2013},

date = {2013-01-01},

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

volume = {82},

number = {10},

abstract = {The low-temperature properties of YbCo2Zn20, which shows a giant specific heat at low temperatures, have been studied by the 59Co-nuclear quadrupole resonance (NQR) technique. The measurement of spin lattice relaxation rate reveals that Yb 4f-electrons unusually persist in a well-localized regime down to at least 0.3K without ordering magnetically. With further lowering temperature, NQR frequency decreases below 0.2K reflecting the lowtemperature Fermi liquid state, even suggesting a crossover to an intermediate valence state in close proximity to the localized-delocalzied transition. We also compare the observed unique properties of YbCo 2Zn20 with those of YbRh2Si2, which shows antiferromagnetic ordering at extremely low temperature. © 2013 The Physical Society of Japan.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Hattori2013,

title = {Spin susceptibility in the superconducting state of the ferromagnetic superconductor UCoGe},

author = {T Hattori and K Karube and Y Ihara and K Ishida and K Deguchi and N K Sato and T Yamamura},

doi = {10.1103/PhysRevB.88.085127},

year  = {2013},

date = {2013-01-01},

journal = {Physical Review B - Condensed Matter and Materials Physics},

volume = {88},

number = {8},

abstract = {In order to determine the superconducting paring state in the ferromagnetic superconductor UCoGe, the 59Co NMR Knight shift, which is directly related to the microscopic spin susceptibility, was measured in the superconducting state under magnetic fields perpendicular to the spontaneous magnetization axis 59Ka,b. 59Ka,b shows to be almost constant, but does not decrease below a superconducting transition. These behaviors as well as the invariance of the internal field at the Co site in the superconducting state exclude the spin-singlet pairing, and can be interpreted with the equal-spin pairing state with a large exchange field along the c axis, which was studied by Mineev. © 2013 American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Karube2013,

title = {Spin-density-wave-type ordering of LaCoGe revealed by 59Co-and 139La-nuclear magnetic resonance measurements},

author = {K Karube and T Hattori and K Ishida and N Tamura and K Deguchi and N K Sato},

doi = {10.7566/JPSJ.82.083712},

year  = {2013},

date = {2013-01-01},

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

volume = {82},

number = {8},

abstract = {The low-temperature magnetic properties of LaCoGe with the tetragonal CeFeSi-type structure were investigated by 59Co- and 139La-nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements. The nuclear spin-lattice relaxation rate divided by the temperature},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Nakai2013,

title = {Normal-state spin dynamics in the iron-pnictide superconductors BaFe 2(As1-xPx)2 and Ba(Fe 1-xCox)2As2 probed with NMR measurements},

author = {Y Nakai and T Iye and S Kitagawa and K Ishida and S Kasahara and T Shibauchi and Y Matsuda and H Ikeda and T Terashima},

doi = {10.1103/PhysRevB.87.174507},

year  = {2013},

date = {2013-01-01},

journal = {Physical Review B - Condensed Matter and Materials Physics},

volume = {87},

number = {17},

abstract = {The NMR results in iron pnictides BaFe2(As1-xP x)2 and Ba(Fe1-xCox) 2As2 are analyzed based on the self-consistent renormalization (SCR) spin-fluctuation theory. The temperature dependence of the NMR relaxation rate T1-1 as well as the electrical resistivity is well reproduced by a SCR model where two-dimensional antiferromagnetic (AF) spin fluctuations are dominant. The successful description of the crossover feature from non-Fermi-liquid to Fermi-liquid behavior strongly suggests that low-lying spin fluctuations in BaFe2(As1-xPx)2 and Ba(Fe1-xCox)2As2 possess an itinerant AF nature, and that chemical substitution in the two compounds tunes the distance of these systems to an AF quantum critical point. The close relationship between spin fluctuations and superconductivity is discussed compared with the other unconventional superconductors, cuprate and heavy fermion superconductors. In addition, it is suggested that magnetism and lattice instability in these pnictides are strongly linked via orbital degrees of freedom. © 2013 American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kitagawa2013,

title = {Ferromagnetic quantum critical point induced by tuning the magnetic dimensionality of the heavy-fermion iron oxypnictide Ce(Ru1-xFe x)PO},

author = {S Kitagawa and K Ishida and T Nakamura and M Matoba and Y Kamihara},

doi = {10.7566/JPSJ.82.033704},

year  = {2013},

date = {2013-01-01},

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

volume = {82},

number = {3},

abstract = {We have performed 31P-NMR measurements of the c-axis-aligned poly-crystal Ce(Ru1-xFex)PO with a twodimensional layered structure in order to understand the origin of TCurie suppression by Fe substitution. The variation in the magnetic-fluctuation character with respect to x is investigated from the in-plane and out-of-plane fluctuations and the relationship between the static spin susceptibility and the in-plane fluctuation. It was found that three-dimensional ferromagnetic (FM) correlations are dominant and give rise to the FM ordering in CeRuPO. The out-of plane fluctuations are significantly suppressed with increasing x, and it was revealed that the FM fluctuations become two dimensional near a FM quantum critical point (QCP). Our NMR results strongly suggest that a unique FM QCP in Ce(Ru 1-xFex)PO is induced by the suppression of the magnetic correlations along the c-axis, which is a different mechanism from that presumed in heavy-fermion compounds. © 2013 The Physical Society of Japan.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kitagawa2013b,

title = {S-wave superconductivity in superconducting BaTi2Sb2O revealed by 121/123Sb-NMR/nuclear quadrupole resonance measurements},

author = {S Kitagawa and K Ishida and K Nakano and T Yajima and H Kageyama},

doi = {10.1103/PhysRevB.87.060510},

year  = {2013},

date = {2013-01-01},

journal = {Physical Review B - Condensed Matter and Materials Physics},

volume = {87},

number = {6},

abstract = {We report the 121/123Sb-NMR/nuclear quadrupole resonance (NQR) measurements on the superconductor BaTi2Sb2O with a two-dimensional Ti2O square-net layer formed with Ti3+ (3d1). NQR measurements revealed that the in-plane four-fold symmetry is broken at the Sb site below TA∼40 K, without an internal field appearing at the Sb site. These exclude a spin-density wave (SDW)/ charge density wave (CDW) ordering with incommensurate correlations, but can be understood with the commensurate CDW ordering at TA. The spin-lattice relaxation rate 1/T1, measured at the four-fold symmetry breaking site, decreases below superconducting (SC) transition temperature Tc, indicative of the microscopic coexistence of superconductivity and the CDW/SDW phase below TA. Furthermore, 1/T1 of 121Sb-NQR shows a coherence peak just below Tc and decreases exponentially at low temperatures. These results are in sharp contrast with those in cuprate and iron-based superconductors, and strongly suggest that its SC symmetry is classified to an ordinary s-wave state. © 2013 American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Maeno2012,

title = {Evaluation of spin-triplet superconductivity in Sr 2RuO 4},

author = {Y Maeno and S Kittaka and T Nomura and S Yonezawa and K Ishida},

doi = {10.1143/JPSJ.81.011009},

year  = {2012},

date = {2012-01-01},

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

volume = {81},

number = {1},

abstract = {This review presents a summary and evaluations of the superconducting properties of the layered ruthenate Sr 2RuO 4 as they are known in the autumn of 2011. This paper appends the main progress that has been made since the preceding review by Mackenzie and Maeno was published in 2003. Here, special focus is placed on the critical evaluation of the spin-triplet, odd-parity pairing scenario applied to Sr 2RuO 4. After an introduction to superconductors with possible odd-parity pairing, accumulated evidence for the pairing symmetry of Sr 2RuO 4 is examined. Then, significant recent progress on the theoretical approaches to the superconducting pairing by Coulomb repulsion is reviewed. A section is devoted to some experimental properties of Sr 2RuO 4 that seem to defy simple explanations in terms of currently available spin-triplet scenario. The next section deals with some new developments using eutectic boundaries and micro-crystals, which reveals novel superconducting phenomena related to chiral edge states, odd-frequency pairing states, and half-fluxoid states. Some of these properties are intimately connected with the properties as a topological superconductor. The article concludes with a summary of knowledge emerged from the study of Sr 2RuO 4 that are now more widely applied to understand the physics of other unconventional superconductors, as well as with a brief discussion of relatively unexplored but promising areas of ongoing and future studies of Sr 2RuO 4. © 2012 The Physical Society of Japan.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Deguchi20121013,

title = {Quantum critical state in a magnetic quasicrystal},

author = {K Deguchi and S Matsukawa and N K Sato and T Hattori and K Ishida and H Takakura and T Ishimasa},

doi = {10.1038/nmat3432},

year  = {2012},

date = {2012-01-01},

journal = {Nature Materials},

volume = {11},

number = {12},

pages = {1013-1016},

abstract = {Quasicrystals are metallic alloys that possess long-range, aperiodic structures with diffraction symmetries forbidden to conventional crystals. Since the discovery of quasicrystals by Schechtman et al. in 1984, there has been considerable progress in resolving their geometric structure. For example, it is well known that the golden ratio of mathematics and art occurs over and over again in their crystal structure. However, the characteristic properties of the electronic states - whether they are extended as in periodic crystals or localized as in amorphous materials - are still unresolved. Here we report the first observation of quantum (T=0) critical phenomena of the Au-Al-Yb quasicrystal - the magnetic susceptibility and the electronic specific heat coefficient arising from strongly correlated 4f electrons of the Yb atoms diverge as T→0. Furthermore, we observe that this quantum critical phenomenon is robust against hydrostatic pressure. By contrast, there is no such divergence in a crystalline approximant, a phase whose composition is close to that of the quasicrystal and whose unit cell has atomic decorations (that is, icosahedral clusters of atoms) that look like the quasicrystal. These results clearly indicate that the quantum criticality is associated with the unique electronic state of the quasicrystal, that is, a spatially confined critical state. Finally we discuss the possibility that there is a general law underlying the conventional crystals and the quasicrystals. © 2012 Macmillan Publishers Limited. All rights reserved.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kitagawa2012,

title = {Ferromagnetic quantum critical point in heavy-fermion iron oxypnictide Ce(Ru1-xFex)PO},

author = {S Kitagawa and K Ishida and T Nakamura and M Matoba and Y Kamihara},

doi = {10.1103/PhysRevLett.109.227004},

year  = {2012},

date = {2012-01-01},

journal = {Physical Review Letters},

volume = {109},

number = {22},

abstract = {We have performed P31-NMR measurements on Ce(Ru1-xFe x)PO in order to investigate ferromagnetic (FM) quantum criticality, since a heavy-fermion (HF) ferromagnet CeRuPO with a two-dimensional structure turns into a HF paramagnet by an isovalent Fe substitution for Ru. We found that Ce(Ru0.15Fe0.85)PO shows critical fluctuations down to ∼0.3K, as well as the continuous suppression of Curie temperature and the ordered moments by the Fe substitution. These experimental results suggest the presence of a FM quantum critical point (QCP) at x∼0.86, which is a rare example among itinerant ferromagnets. In addition, we point out that the critical behaviors in Ce(Ru0.15Fe0.85)PO share a similarity with those in YbRh2Si2, where the local criticality of f electrons has been discussed. We reveal that Ce(Ru 1-xFex)PO is a new system to study FM quantum criticality in HF compounds. © 2012 American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Karube2012,

title = {Universality and critical behavior at the critical endpoint in the itinerant-electron metamagnet UCoAl},

author = {K Karube and T Hattori and S Kitagawa and K Ishida and N Kimura and T Komatsubara},

doi = {10.1103/PhysRevB.86.024428},

year  = {2012},

date = {2012-01-01},

journal = {Physical Review B - Condensed Matter and Materials Physics},

volume = {86},

number = {2},

abstract = {We performed nuclear-magnetic-resonance measurements on itinerant-electron metamagnet UCoAl to investigate the critical behavior of the magnetism near a metamagnetic (MM) critical endpoint (CEP). We derived c-axis magnetization M c and its fluctuation S c from the measurements of Knight shift and nuclear spin-lattice relaxation rate 1/T 1 as a function of the c-axis external field (H c) and temperature (T). We developed contour plots of M c and S c on the H c-T phase diagram, and observed the strong divergence of S c at the CEP. The critical exponents of M c and S c near the CEP are estimated and found to be close to the universal properties of a three-dimensional Ising model. We indicate that the critical phenomena at the itinerant-electron MM CEP in UCoAl have a common feature as a gas-liquid transition. © 2012 American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Iye2012,

title = {Gradual suppression of antiferromagnetism in BaFe 2(As 1-xP x) 2: Zero-temperature evidence for a quantum critical point},

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

doi = {10.1103/PhysRevB.85.184505},

year  = {2012},

date = {2012-01-01},

journal = {Physical Review B - Condensed Matter and Materials Physics},

volume = {85},

number = {18},

abstract = {Static and dynamic magnetic properties of lightly P-substituted BaFe 2(As 1-xP x) 2 were systematically investigated by 31P NMR. The averaged internal magnetic field at the P site in the zero-temperature limit evaluated from the broadening of NMR spectra in the antiferromagnetic (AFM) phase is gradually suppressed toward x∼0.35 with increasing x, which provides definitive evidence for the existence of an AFM quantum critical point (QCP) at x∼0.35. The location of the AFM QCP is consistent with the previous estimation from temperature dependence of spin dynamics in the normal state, and the superconducting transition temperature T c takes the maximum around the QCP. Our experiments, revealing a signature of a QCP extending up to room temperature, establish BaFe 2(As 1-xP x) 2 as one of the most accessible systems for unraveling the nature of quantum criticality and the relationship between AFM quantum criticality and unconventional superconductivity. © 2012 American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Nakai2012,

title = {Enhanced anisotropic spin fluctuations below tetragonal-to-orthorhombic transition in LaFeAs(O 1-xF x) probed by 75As and 139La NMR},

author = {Y Nakai and S Kitagawa and T Iye and K Ishida and Y Kamihara and M Hirano and H Hosono},

doi = {10.1103/PhysRevB.85.134408},

year  = {2012},

date = {2012-01-01},

journal = {Physical Review B - Condensed Matter and Materials Physics},

volume = {85},

number = {13},

abstract = {75As and 139La NMR results for LaFeAs(O 1-xF x) (x=0, 0.025, and 0.04) are reported. Upon F doping, the tetragonal-to-orthorhombic structural phase transition temperature T S, antiferromagnetic transition temperature T N, and internal magnetic field μ 0H int are gradually reduced for x<0.04. However, at x=0.04, T N is abruptly suppressed to be 30 K along with a tiny μ 0H int, which is distinct from the continuous disappearance of the ordered phases in the Ba122 systems of Ba(Fe,Co) 2As 2 and BaFe 2(As,P) 2. The anisotropy of the spin-lattice relaxation rate T1-1, (T 1)H∥ab-1/ (T 1)H∥c-1, in the paramagnetic phase of x=0 and 0.025 is constant (∼1.5) but increases abruptly below T S due to the enhancement of (T 1)H∥ab-1 by the slowing-down of magnetic fluctuations. This indicates that the tetragonal-to-orthorhombic structural distortion enhances the anisotropy in the spin space via magnetoelastic coupling and/or spin-orbit interaction. © 2012 American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Iye2012b,

title = {Microscopic evidence of direct coupling between magnetic and superconducting order parameters in BaFe 2(As 1-xP x) 2},

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

doi = {10.1143/JPSJ.81.033701},

year  = {2012},

date = {2012-01-01},

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

volume = {81},

number = {3},

abstract = {The coexistence of magnetism and superconductivity in the isovalent-P-substituted BaFe 2(As 1-xP x) 2 has been investigated microscopically by 31P-NMR measurements. We found that superconducting (SC) transition occurs in a magnetic region with static ordered moments and that the moments are reduced below the SC transition temperature T c in the samples near the phase boundary of magnetism and superconductivity. Our results indicate that magnetism and superconductivity coexist spatially but compete with each other on the same Fermi surfaces. The coexistence state is qualitatively different from that observed in other unconventional superconductors and gives a strict constraint on the theoretical model for superconductivity in BaFe 2(As 1-xP x) 2. © 2012 The Physical Society of Japan.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Hattori2012,

title = {Superconductivity induced by longitudinal ferromagnetic fluctuations in UCoGe},

author = {T Hattori and Y Ihara and Y Nakai and K Ishida and Y Tada and S Fujimoto and N Kawakami and E Osaki and K Deguchi and N K Sato and I Satoh},

doi = {10.1103/PhysRevLett.108.066403},

year  = {2012},

date = {2012-01-01},

journal = {Physical Review Letters},

volume = {108},

number = {6},

abstract = {From detailed angle-resolved NMR and Meissner measurements on a ferromagnetic (FM) superconductor UCoGe (T Curie∼2.5K and T SC∼0.6K), we show that superconductivity in UCoGe is tightly coupled with longitudinal FM spin fluctuations along the c axis. We found that magnetic fields along the c axis (Hc) strongly suppress the FM fluctuations and that the superconductivity is observed in the limited magnetic-field region where the longitudinal FM spin fluctuations are active. These results, combined with model calculations, strongly suggest that the longitudinal FM spin fluctuations tuned by Hc induce the unique spin-triplet superconductivity in UCoGe. This is the first clear example that FM fluctuations are intimately related with superconductivity. © 2012 American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Ishida2012b,

title = {Preface},

author = {K Ishida and M Ogata},

year  = {2012},

date = {2012-01-01},

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

volume = {81},

number = {1},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kitagawa2011,

title = {Metamagnetic behavior and Kondo breakdown in heavy-fermion CeFePO},

author = {S Kitagawa and H Ikeda and Y Nakai and T Hattori and K Ishida and Y Kamihara and M Hirano and H Hosono},

doi = {10.1103/PhysRevLett.107.277002},

year  = {2011},

date = {2011-01-01},

journal = {Physical Review Letters},

volume = {107},

number = {27},

abstract = {We report that nonmagnetic heavy-fermion (HF) iron oxypnictide CeFePO with two-dimensional XY-type anisotropy shows a metamagnetic behavior at the metamagnetic field H M4T perpendicular to the c axis and that a critical behavior is observed around H M. Although the magnetic character is entirely different from that in other Ce-based HF metamagnets, H M in these metamagnets is linearly proportional to the inverse of the effective mass, or to the temperature where the susceptib+ility shows a peak. This finding suggests that H M is a magnetic field breaking the local Kondo singlet, and the critical behavior around H M is driven by the Kondo breakdown accompanied by the Fermi-surface instability. © 2011 American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kii2011897,

title = {Conceptual design of a novel insertion device using bulk superconducting magnet},

author = {T Kii and R Kinjo and M A Bakr and Y W Choi and K Yoshida and S Ueda and M Takasaki and K Ishida and N Kimura and T Sonobe and K Masuda and H Ohgaki},

doi = {10.1016/j.physc.2011.05.083},

year  = {2011},

date = {2011-01-01},

journal = {Physica C: Superconductivity and its Applications},

volume = {471},

number = {21-22},

pages = {897-900},

abstract = {An undulator or a wiggler with a strong magnetic field will play an important role in future synchrotron light sources, free electron lasers, and linear colliders. We proposed the bulk high critical temperature superconductor staggered array undulator (Bulk HTSC SAU) in order to generate a strong periodic field. The Bulk HTSC SAU consists of stacked bulk high-Tc superconductors (HTSs) and a solenoid magnet which is used to magnetize the bulk HTSs. A periodic magnetic field was produced and controlled using a prototype of the Bulk HTSC SAU using 11 pairs of REBaCuO bulk HTSs at 77 K. The expected performance at low temperatures around 20 K is calculated using a loop current model. © 2011 Elsevier B.V. All rights reserved.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Hattori2011,

title = {59Co-NMR/nuclear-quadrupole-resonance (NQR) studies on ferromagnetic superconductor UCoGe},

author = {T Hattori and Y Ihara and K Ishida and Y Nakai and E Osaki and K Deguchi and N K Sato and I Satoh},

doi = {10.1143/JPSJS.80SA.SA007},

year  = {2011},

date = {2011-01-01},

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

volume = {80},

number = {SUPPL. A},

abstract = {We have performed NMR and nuclear-quadrupole-resonance (NQR) on U-based ferromagnetic superconductor UCoGe and a reference metallic compound YCoGe without 5f electrons, in order to investigate the normal and superconducting-state properties. From 59Co-NQR measurements, we give clear evidence that ferromagnetism and superconductivity coexist microscopically, both of which are ascribed to the U 5 f-electrons. In addition, direction-dependent Knight shift and nuclear spin-lattice relaxation rate measurements provide microscopic evidence that both static and dynamic susceptibilities are ferromagnetic with strong Ising anisotropy. We point out that superconductivity induced by such magnetic fluctuations prefers spin-triplet pairing state © 2011 The Physical Society of Japan.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Bakr2011,

title = {Back bombardment for dispenser and lanthanum hexaboride cathodes},

author = {M Bakr and R Kinjo and Y W Choi and M Omer and K Yoshida and S Ueda and M Takasaki and K Ishida and N Kimura and T Sonobe and T Kii and K Masuda and H Ohgaki and H Zen},

doi = {10.1103/PhysRevSTAB.14.060708},

year  = {2011},

date = {2011-01-01},

journal = {Physical Review Special Topics - Accelerators and Beams},

volume = {14},

number = {6},

abstract = {The back bombardment (BB) effect limits wide usage of thermionic rf guns. The BB effect induces not only ramping-up of a cathode's temperature and beam current, but also degradation of cavity voltage and beam energy during a macropulse. This paper presents a comparison of the BB effect for the case of dispenser tungsten-base (DC) and lanthanum hexaboride (LaB6) thermionic rf gun cathodes. For each, particle simulation codes are used to simulate the BB effect and electron beam dynamics in a thermionic rf gun cathode. A semiempirical equation is also used to investigate the stopping range and deposited heat power of BB electrons in the cathode material. A numerical simulation method is used to calculate the change of the cathode temperature and current density during a single macropulse. This is done by solving two differential equations for the rf gun cavity equivalent circuit and one-dimensional thermal diffusion equation. High electron emission and small beam size are required for generation of a high-brightness electron beam, and so in this work the emission properties of the cathode are taken into account. Simulations of the BB effect show that, for a pulse of 6μs duration, the DC cathode experiences a large change in the temperature compared with LaB 6, and a change in current density 6 times higher. Validation of the simulation results is performed using experimental data for beam current beyond the gun exit. The experimental data is well reproduced using the simulation method. © 2011 American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}