北川 俊作

@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.},

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{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{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{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{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{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{Ishida2011515,

title = {NMR studies on iron-pnictide superconductors: LaFeAs(O1-xFx) and BaFe2(As1-xPx)2},

author = {K Ishida and Y Nakai and S Kitagawa and T Iye},

doi = {10.1016/j.crhy.2011.04.004},

year  = {2011},

date = {2011-01-01},

journal = {Comptes Rendus Physique},

volume = {12},

number = {5-6},

pages = {515-531},

abstract = {This review presents a summary of our NMR experiments on LaFeAs(O1-xFx) and BaFe2(As1-xPx)2. The authors introduce the key NMR experimental results which reveal their physical properties in the normal and superconducting states. From the comparison of the NMR results between the two systems, we point out that the superconducting gap structure and the relationship between superconductivity and magnetic fluctuations probed with NMR are quite different between the two systems. © 2011 Académie des sciences.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kitagawa2011b,

title = {Nonmagnetic pair-breaking effect in La(Fe1-xZn x)AsO0.85 studied by As75 and La139 NMR and NQR},

author = {S Kitagawa and Y Nakai and T Iye and K Ishida and Y F Guo and Y G Shi and K Yamaura and E Takayama-Muromachi},

doi = {10.1103/PhysRevB.83.180501},

year  = {2011},

date = {2011-01-01},

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

volume = {83},

number = {18},

abstract = {As75 and La139 NMR and nuclear quadrupole resonance (NQR) studies on Zn-substituted LaFeAsO0.85 have been performed to microscopically investigate the Zn-impurity effects. Although superconductivity in LaFeAsO 0.85 disappears by 3% Zn substitution, we found that NMR/NQR spectra and NMR physical quantities in the normal state are hardly changed, indicating that the crystal structure and electronic states are not modified by Zn substitution. Our results suggest that the suppression of superconductivity by Zn substitution is not due to the change of the normal-state properties, but due to strong nonmagnetic pair-breaking effect to superconductivity. © 2011 American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kitagawa2010,

title = {F-doping dependence of 75As nuclear quadrupole resonance frequency in LaFeAs(O1-xFx)},

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

doi = {10.1016/j.physc.2009.10.106},

year  = {2010},

date = {2010-01-01},

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

volume = {470},

number = {SUPPL.1},

pages = {S282-S283},

abstract = {We report the F-doping dependence of 75As nuclear quadruple resonance frequency (75νQ) in the iron-oxypnictide LaFeAs(O1-xFx) (x = 0, 0.07, 0.08, 0.11 and 0.14). It was revealed that 75νQ becomes larger with increasing x up to 0.14. From point charge calculations, we found that experimental F-doping dependence of 75νQ cannot be accounted for by the effect of surrounding ions. Therefore, the on-site charge distribution at the 75As site modified by the hybridization between As-4p and Fe-3d orbitals gives dominant contribution to 75νQ in LaFeAs(O1-xFx). © 2009 Elsevier B.V. All rights reserved.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Nakai2010,

title = {Antiferromagnetic fluctuations in iron pnictide superconductor BaFe 2(As0.67P0.33)2 investigated by 31P NMR},

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

doi = {10.1016/j.physc.2010.01.047},

year  = {2010},

date = {2010-01-01},

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

volume = {470},

number = {SUPPL.1},

pages = {S420-S421},

abstract = {31P NMR was performed on the normal state in the newly discovered iron pnictide superconductor BaFe2(As0.67P 0.33)2 with maximum Tc of 30 K. The Knight shift is temperature-independent while the nuclear spin-lattice relaxation rate divided by temperature (T1T)-1 exhibits a more than twofold increase on cooling. This contrasting behavior clearly indicates the development of low-lying antiferromagnetic fluctuations in BaFe2(As 0.67P0.33)2 as observed in (Ba 1-xKx)Fe2As2. The observed low-lying antiferromagnetic fluctuations may be universal in optimally doped "122" superconductors, in contrast to "1111" superconductors. © 2009 Elsevier B.V. All rights reserved.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Nakai2010b,

title = {Unconventional superconductivity and antiferromagnetic quantum critical behavior in the isovalent-doped BaFe2(As1-xP x)2},

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

doi = {10.1103/PhysRevLett.105.107003},

year  = {2010},

date = {2010-01-01},

journal = {Physical Review Letters},

volume = {105},

number = {10},

abstract = {Spin dynamics evolution of BaFe2(As1-xP x)2 was probed as a function of P concentration via P31 NMR. Our NMR study reveals that two-dimensional antiferromagnetic (AF) fluctuations are notably enhanced with little change in static susceptibility on approaching the AF phase from the superconducting dome. Moreover, the magnetically ordered temperature θ deduced from the relaxation rate vanishes at optimal doping. These results provide clear-cut evidence for a quantum-critical point, suggesting that the AF fluctuations associated with the quantum-critical point play a central role in the high-Tc superconductivity.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kitagawa2010b,

title = {Stripe antiferromagnetic correlations in LaFeAsO1-x Fx probed by A 75 s NMR},

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

doi = {10.1103/PhysRevB.81.212502},

year  = {2010},

date = {2010-01-01},

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

volume = {81},

number = {21},

abstract = {The anisotropy of the nuclear spin-lattice relaxation rate 1/T1 of A 75 As was investigated in the iron pnictide LaFeAs (O 1-xFx) (x=0.07, 0.11, and 0.14) as well as LaFeAsO. While the temperature dependence of the normal-state 1/T1T in the superconducting (SC) x=0.07 is different from that in the SC x=0.11, their anisotropy of 1/T1, R≡(1/T1)H∥ab/(1/ T1)H∥c in the normal state is almost the same (∼1.5). The observed anisotropy is ascribable to the presence of the local stripe correlations with Q= (π,0) or (0,π). In contrast, 1/ T1 is isotropic and R is approximately 1 in the overdoped x=0.14 sample, where superconductivity is almost suppressed. These results suggest that the presence of the local stripe correlations originating from the nesting between hole and electron Fermi surfaces is linked to high- Tc superconductivity in iron pnictides. © 2010 The American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Nakai2010c,

title = {Erratum: Systematic 75As NMR study of the dependence of low-lying excitations on F doping in the iron oxypnictide LaFeAs O1-x Fx(Physical Review B (2009) 79 (212506))},

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

doi = {10.1103/PhysRevB.81.019902},

year  = {2010},

date = {2010-01-01},

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

volume = {81},

number = {1},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Nakai2010d,

title = {P 31 and A 75 s NMR evidence for a residual density of states at zero energy in superconducting BaFe2( As0.67P 0.33)2},

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

doi = {10.1103/PhysRevB.81.020503},

year  = {2010},

date = {2010-01-01},

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

volume = {81},

number = {2},

abstract = {P 31 and A 75 s NMR measurements were performed in superconducting BaFe2( As0.67P0.33)2 with Tc =30 K. The nuclear-spin-lattice relaxation rate T 1 -1 and the Knight shift in the normal state indicate the development of antiferromagnetic fluctuations, and T 1 -1 in the superconducting (SC) state decreases without a coherence peak just below Tc, as observed in (Ba1-x Kx) Fe2 As 2. In contrast to other iron arsenide superconductors, the T 1 -1 T behavior is observed below 4 K, indicating the presence of a residual density of states at zero energy. Our results suggest that strikingly different SC gaps appear in BaFe2 (As1-x Px)2despite a comparable Tc value, an analogous phase diagram, and similar Fermi surfaces to (Ba1-x Kx) Fe2 As2. © 2010 The American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Ishida20093089,

title = {75As and 139La NMR studies on LaFeAs (O1 - x Fx)},

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

doi = {10.1016/j.physb.2009.07.078},

year  = {2009},

date = {2009-01-01},

journal = {Physica B: Condensed Matter},

volume = {404},

number = {19},

pages = {3089-3094},

abstract = {We report experimental results of 75As and 139La nuclear magnetic resonance (NMR) in the layered oxypnictide system LaFeAs (O1 - x Fx) (x = 0.0, 0.04, 0.07 and 0.11) where superconductivity occurs in x greater than 0.04. In the undoped LaFeAsO, 1 / T1 of 139La exhibits a distinct peak at TN ~ 142 K below which the La NMR spectra become broadened due to the internal magnetic field attributed to an antiferromagnetic (AFM) ordering. In the x = 0.04 sample, 1 / T1 T of 75As exhibits a Curie-Weiss temperature dependence down to 30 K, suggesting the development of AFM spin fluctuations with decreasing temperature. In the x = 0.11 sample, in contrast, pseudogap behavior is observed in 1 / T1 T at the 75As site with a gap value of ΔPG ~ 175 K. The spin dynamics vary markedly with F-doping, which is ascribed to the change of the Fermi-surface structure by the electron-doping. As for the superconducting properties for x = 0.04, 0.07 and 0.11, 1 / T1 of 75As in all compounds does not exhibit a coherence peak just below Tc and follows a T3 dependence at low temperatures. These results seemingly suggests that unconventional superconductivity with zero gap along lines, whereas the lack of the residual density of states at the low temperatures is incompatible with the presence of the line-nodes. We discuss similarity and difference between LaFeAs (O1 - x Fx) and cuprates, and also discuss the relationship between spin dynamics and superconductivity on the basis of F-doping dependence of Tc and 1 / T1. Crown Copyright © 2009.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Nakai2009,

title = {Systematic A 75 s NMR study of the dependence of low-lying excitations on F doping in the iron oxypnictide LaFeAsO1-x Fx},

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

doi = {10.1103/PhysRevB.79.212506},

year  = {2009},

date = {2009-01-01},

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

volume = {79},

number = {21},

abstract = {We report A 75 s NMR studies on LaFeAs (O1-x Fx) (0≤x≤0.14). At x=0.04 near the phase boundary, from resistivity, spin-lattice relaxation rate 1/ T1, and NMR spectrum measurements, we found weak magnetic order at TN □30 K. Antiferromagnetic (AFM) fluctuations proved through 1/ T1 are suppressed significantly with F doping, and pseudogap behavior without pronounced AFM fluctuations is observed at x=0.11 where TC is maximum. This significant suppression of 1/ T1 T upon F doping while TC remains nearly unchanged suggests that low-energy AFM fluctuations probed with A 75 s NMR do not play a crucial role in the superconductivity. © 2009 The American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Nakai2009b,

title = {An NMR study on the F-doping evolution of the iron oxypnictide LaFeAs(O1-xFx)},

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

doi = {10.1088/1367-2630/11/4/045004},

year  = {2009},

date = {2009-01-01},

journal = {New Journal of Physics},

volume = {11},

abstract = {We report the experimental results of 75As and 139La nuclear magnetic resonance (NMR) in the layered oxypnictide system LaFeAs(O 1−xFx) (x = 0.0, 0.04, 0.07, 0.11 and 0.14) where superconductivity occurs at x > 0.03. In the undoped LaFeAsO, 1/T 1 of 139La exhibits a distinct peak at TiN ∼ 142 K below which the La-NMR spectra become broadened due to the internal magnetic field attributed to an antiferromagnetic (AFM) ordering. In the x = 0.04 sample, 1/T1T of 75As exhibits a Curie-Weiss temperature dependence down to 30 K, suggesting the development of AFM spin fluctuations with a decrease in temperature. At x = 0.11 and 0.14, in contrast, pseudogap behavior is observed in 1/ T1T at the 75As site with a gap value of δPG ∼ 175 and 165 K, respectively. The spin dynamics vary markedly with F-doping, which is ascribed to the change of the Fermi-surface structure because of the electron doping. As for the superconducting properties for x = 0.04, 0.07 and 0.11, the 1 / T1 of75 As in the three samples does not exhibit a coherence peak just below Tc and follows a T3 dependence at low temperatures. These results may suggest unconventional superconductivity with a zero gap along the lines, but neither the field-induced extra relaxation rate nor the residual density of states at the low temperatures is incompatible with the presence of the line nodes. We discuss the similarities and differences between LaFeAs(O1−xFx) and cuprates and also discuss the relationship between spin dynamics and superconductivity on the basis of the F-doping dependence of Tc and 1/T1. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}