Kenji Ishida

@article{Klintberg2010,

title = {Chemical pressure and physical pressure in BaFe2(As 1-xPx)2},

author = {L E Klintberg and S K Goh and S Kasahara and Y Nakai and K Ishida and M Sutherland and T Shibauchi and Y Matsuda and T Terashima},

doi = {10.1143/JPSJ.79.123706},

year  = {2010},

date = {2010-01-01},

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

volume = {79},

number = {12},

abstract = {Measurements of the superconducting transition temperature, Tc, under hydrostatic pressure via bulk AC susceptibility were carried out on several concentrations of phosphorous substitution in BaFe2-(As 1-xPx)2. The pressure dependence of unsubstituted BaFe2As2, phosphorous concentration dependence of BaFe2(As1-xPx)2, as well as the pressure dependence of BaFe2(As1-xP x)2 all point towards an identical maximum Tc of 31 K. This demonstrates that phosphorous substitution and physical pressure result in similar superconducting phase diagrams, and that phosphorous substitution does not induce substantial impurity scattering. © 2010 The Physical Society of Japan.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Ihara2010,

title = {Anisotropic magnetic fluctuations in the ferromagnetic superconductor UCoGe studied by direction-dependent Co59 NMR measurements},

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

doi = {10.1103/PhysRevLett.105.206403},

year  = {2010},

date = {2010-01-01},

journal = {Physical Review Letters},

volume = {105},

number = {20},

abstract = {We have carried out direction-dependent Co59 NMR experiments on a single crystal sample of the ferromagnetic superconductor UCoGe in order to study the magnetic properties in the normal state. The 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 discuss that superconductivity induced by these magnetic fluctuations prefers spin-triplet pairing state. © 2010 The American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Goh2010,

title = {Anisotropic superconducting properties of optimally doped BaFe 2(As0.65P0.35)2 under pressure},

author = {S K Goh and Y Nakai and K Ishida and L E Klintberg and Y Ihara and S Kasahara and T Shibauchi and Y Matsuda and T Terashima},

doi = {10.1103/PhysRevB.82.094502},

year  = {2010},

date = {2010-01-01},

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

volume = {82},

number = {9},

abstract = {Magnetic measurements on optimally doped single crystals of BaFe 2 (As1-xPx)2 (x≈0.35) with magnetic fields applied along different crystallographic axes were performed under pressure, enabling the pressure evolution of coherence lengths and the anisotropy factor to be followed. Despite a decrease in the superconducting critical temperature, our studies reveal that the superconducting properties become more anisotropic under pressure. With appropriate scaling, we directly compare these properties with the values obtained for BaFe2 (As 1-xPx)2 as a function of phosphorus content. © 2010 The American Physical Society.},

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{Ohta2010,

title = {Microscopic coexistence of ferromagnetism and superconductivity in single-crystal UCoGe},

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

doi = {10.1143/JPSJ.79.023707},

year  = {2010},

date = {2010-01-01},

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

volume = {79},

number = {2},

abstract = {Unambiguous evidence for the microscopic coexistence of ferromagnetism and superconductivity in UCoGe (TCurie ∼ 2:5K and TSC ∼ 0:6 K) is reported from 59Co nuclear quadrupole resonance (NQR). The 59Co-NQR signal below 1K indicates ferromagnetism throughout the sample volume, while the nuclear spin-lattice relaxation rate 1/T1 in the ferromagnetic (FM) phase decreases below TSC due to the opening of the superconducting (SC) gap. The SC state is found to be inhomogeneous, suggestive of a self-induced vortex state, potentially realizable in a FM superconductor. In addition, the 59Co-NQR spectrum around TCurie shows that the FM transition in UCoGe possesses a first-order character, which is consistent with the theoretical prediction that the low-temperature FM transition in itinerant magnets is generically of first-order. © 2010 The Physical Society of Japan.},

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{Kitagawa2009,

title = {Erratum: High-temperature spin relaxation process in Dy2 Ti2 O7 probed by Ti47 -NQR},

author = {K Kitagawa and R Higashinaka and K Ishida and Y Maeno and M Takigawa},

doi = {10.1103/PhysRevB.80.229902},

year  = {2009},

date = {2009-01-01},

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

volume = {80},

number = {22},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kittaka2009,

title = {Angular dependence of the upper critical field of Sr2 RuO4},

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

doi = {10.1103/PhysRevB.80.174514},

year  = {2009},

date = {2009-01-01},

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

volume = {80},

number = {17},

abstract = {One of the remaining issues concerning the spin-triplet superconductivity of Sr2 RuO4 is the strong limit of the in-plane upper critical field Hc2 at low temperatures. In this study, we clarified the dependence of Hc2 on the angle θ between the magnetic field and the ab plane at various temperatures, by precisely and accurately controlling the magnetic field direction. We revealed that, although the temperature dependence of Hc2 for |θ| ≥5°is well explained by the orbital pair-breaking effect, Hc2 (T) for |θ| <5°is clearly limited at low temperatures. We also revealed that the Hc2 limit for |θ| <5°is present not only at low temperatures but also at temperatures close to Tc. These features may provide additional hints for clarifying the origin of the Hc2 limit. Interestingly, if the anisotropic ratio in Sr2 RuO4 is assumed to depend on temperature, the observed angular dependence of Hc2 is reproduced better at lower temperature with an effective-mass model for an anisotropic three-dimensional superconductor. We discuss the observed behavior of Hc2 based on existing theories. © 2009 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{Nambu2009,

title = {Structural properties of the two-dimensional triangular antiferromagnet NiGa2 S4},

author = {Y Nambu and R T MacAluso and T Higo and K Ishida and S Nakatsuji},

doi = {10.1103/PhysRevB.79.214108},

year  = {2009},

date = {2009-01-01},

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

volume = {79},

number = {21},

abstract = {Polycrystalline samples of NiGa2 S4+δ (-0.2≤δ≤+0.2) have been synthesized by solid-state reaction. X-ray powder-diffraction measurements indicate that these samples are single phase with a trigonal lattice with a nominal concentration δ at least between -0.12 and +0.16. Rietveld analyses for time-of-flight neutron-diffraction data have revealed that there is no site mixing between Ni and Ga atoms, indicating that the triangular lattice is only formed by Ni atoms. Our susceptibility measurements have clarified that the freezing temperature and the magnitude of hysteresis systematically change with nominal sulfur concentration and form a respective peak and minimum at the nominal NiGa2 S4.04. These results together with our chemical analyses indicate that the actual content of sulfur for the nominal NiGa2 S4.04 should be stoichiometric, namely, 4.00. On the contrary, the Ga nuclear quadrupole resonance (NQR) has suggested the presence of two different Ga sites from the Ga-NQR spectra in both samples with the nominal concentration of NiGa2 S4.00 and NiGa2 S4.04, despite the fact that there is a single crystallographic site for Ga in the unit cell. High-resolution transmission electron microscopy scans conducted over a wide region of a NiGa2 S4.04 single crystal found a clean defect-free in-plane structure. Diffraction patterns of the selected area indicate the partial presence of a superlatticelike stacking fault. This suggests that the two different Ga sites are not generated by the sulfur deficiency but by the superlatticelike stacking fault. © 2009 The American Physical Society.},

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{Ishida2009b,

title = {To what extent iron-pnictide new superconductors have been clarified: A progress report},

author = {K Ishida and Y Nakai and H Hosono},

doi = {10.1143/JPSJ.78.062001},

year  = {2009},

date = {2009-01-01},

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

volume = {78},

number = {6},

abstract = {In this review, the authors present a summary of experimental reports on newly discovered iron-based superconductors as they were known at the end of 2008. At the same time, this paper is intended to be useful for experimenters to know the current status of these superconductors. The authors introduce experimental results that reveal basic physical properties in the normal and superconducting states. The similarities and differences between iron-pnictide superconductors and other unconventional superconductors are also discussed. © 2009 The Physical Society of Japan.},

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}

}

@article{MacLaughlin2009667,

title = {Weak quasistatic magnetism in the frustrated Kondo lattice Pr2Ir2O7},

author = {D E MacLaughlin and Y Ohta and Y Machida and S Nakatsuji and G M Luke and K Ishida and R H Heffner and L Shu and O O Bernal},

doi = {10.1016/j.physb.2008.11.167},

year  = {2009},

date = {2009-01-01},

journal = {Physica B: Condensed Matter},

volume = {404},

number = {5-7},

pages = {667-670},

abstract = {Muon spin relaxation experiments have been performed in the pyrochlore iridate Pr2Ir2O7 for temperatures in the range 0.025-250 K. Kubo-Toyabe relaxation functions are observed up to ≳ 200 K, indicating static magnetism over this temperature range. The T → 0 static muon spin relaxation rate Δ (0) ≈ 8 μ s- 1 implies a weak quasistatic moment (∼ 0.1 μB). The temperature dependence of Δ is highly nonmean-field-like, decreasing smoothly by orders of magnitude but remaining nonzero for T < Tf. The data rule out ordering of the full Pr3 + CEF ground-state moment (3.0 μB) down to 0.025 K. The weak static magnetism is most likely due to hyperfine-enhanced 141Pr nuclear magnetism. The dynamic relaxation rate λ increases markedly below ∼ 20 K, probably due to slowing down of spin fluctuations in the spin-liquid state. At low temperatures λ is strong and temperature-independent, indicative of a high density of low-lying spin excitations as is common in frustrated antiferromagnets. © 2008 Elsevier B.V.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Ihara2009,

title = {Magnetic anomalies of hydrous cobaltate compound Nax(H 3O)zCoO2·yH2O detected by NMR and NQR measurements},

author = {Y Ihara and H Takeya and K Ishida and K Yoshimura and K Takada and T Sasaki and H Sakurai and E Takayama-Muromachi},

doi = {10.1103/PhysRevB.79.024510},

year  = {2009},

date = {2009-01-01},

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

volume = {79},

number = {2},

abstract = {In order to investigate the relationship between superconductivity and magnetism in bilayer-hydrate cobaltate Nax (H3 O) z CoO2·y H2 O Co nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements were performed on three different samples, which demonstrate various ground states at low temperatures. The appearance of small internal fields is observed in the NQR spectra below approximately 6 K on one of the samples that possesses the largest c -axis length and the highest NQR frequency. The other two samples exhibit superconducting transition in zero magnetic field, while these two samples show different ground states in the magnetic fields greater than 5 T. The comparison of the NMR spectra of these two samples obtained in high magnetic fields reveals the appearance of static internal magnetic fields at the Co site below 4 K in the sample that possesses the intermediate c -axis length and the NQR frequency. © 2009 The American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Ishida20083108,

title = {NMR and NQR studies on superconducting Sr2RuO4},

author = {K Ishida and H Murakawa and H Mukuda and Y Kitaoka and Z Q Mao and Y Maeno},

doi = {10.1016/j.jpcs.2008.06.028},

year  = {2008},

date = {2008-01-01},

journal = {Journal of Physics and Chemistry of Solids},

volume = {69},

number = {12},

pages = {3108-3114},

abstract = {We review our nuclear-magnetic resonance (NMR) and nuclear-quadrupole-resonance (NQR) studies in superconducting Sr2RuO4, which have been performed in order to investigate the gap structure and the pairing symmetry in the superconducting state and magnetic fluctuations in the normal state. The spin-lattice relaxation rate (1 / T1) of a high-quality sample with Tc ∼ 1.5 K shows a sharp decrease without a coherence peak just below Tc, followed by a T3 behavior down to 0.15 K. This result indicates that the superconducting gap in pure Sr2RuO4 is a highly anisotropic character with a line-node gap. The Knight shift, which is related to the spin susceptibility, is unchanged in the superconducting state irrespective of the direction of the applied fields and various magnitude of the field. This result strongly suggests that the superconducting pairs are in the spin-triplet state, and the spin direction of the triplet pairs is considered to be changed by small fields of several hundred Oe. © 2008 Elsevier Ltd. All rights reserved.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kitagawa2008,

title = {High-temperature spin relaxation process in Dy2 Ti2 O7 probed by T 47 i-NQR},

author = {K Kitagawa and R Higashinaka and K Ishida and Y Maeno and M Takigawa},

doi = {10.1103/PhysRevB.77.214403},

year  = {2008},

date = {2008-01-01},

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

volume = {77},

number = {21},

abstract = {We have performed nuclear quadrupole resonance (NQR) experiments on T 47 i nuclei in Dy2 Ti2 O7 in the temperature range of 70-300 K in order to investigate the dynamics of 4f electrons with strong Ising anisotropy. A significant change in the NQR frequency with temperature was attributed to the variation of the quadrupole moment of Dy4f electrons. A quantitative account was given by the mean-field analysis of the quadrupole-quadrupole (Q-Q) interaction in the presence of the crystalline-electric-field splitting. The magnitude and the temperature dependence of the nuclear spin-lattice relaxation rate were analyzed, including both the spin-spin and the Q-Q interactions. The results indicate that these two types of interaction contribute almost equally to the fluctuation of Dy magnetic moments. © 2008 The American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Takeya2008,

title = {Spin dynamics and spin freezing behavior in the two-dimensional antiferromagnet Ni Ga2 S4 revealed by Ga-NMR, NQR and μsR measurements},

author = {H Takeya and K Ishida and K Kitagawa and Y Ihara and K Onuma and Y Maeno and Y Nambu and S Nakatsuji and D E MacLaughlin and A Koda and R Kadono},

doi = {10.1103/PhysRevB.77.054429},

year  = {2008},

date = {2008-01-01},

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

volume = {77},

number = {5},

abstract = {We have performed Ga 69,71 nuclear magnetic resonance (NMR), nuclear quadrupole resonance (NQR), and muon spin rotation and resonance on the quasi-two-dimensional antiferromagnet Ni Ga2 S4, in order to investigate its spin dynamics and magnetic state at low temperatures. Although there exists only one crystallographic site for Ga in Ni Ga2 S4, we found two distinct Ga signals by NMR and NQR. The origin of the two Ga signals is not fully understood, but possibly due to stacking faults along the c axis which induce additional broad Ga NMR and NQR signals with different local symmetries. We found the spin freezing occurring at Tf, at which the specific heat shows a maximum, from a clear divergent behavior of the nuclear spin-lattice relaxation rate 1/T1 and nuclear spin-spin relaxation rate 1/T2 measured by Ga-NQR as well as the muon spin relaxation rate λ. The main sharp NQR peaks exhibit a stronger tendency of divergence, compared with the weak broader spectral peaks, indicating that the spin freezing is intrinsic in Ni Ga2 S4. The behavior of these relaxation rates strongly suggests that the Ni spin fluctuations slow down towards Tf, and the temperature range of the divergence is anomalously wider than that in a conventional magnetic ordering. A broad structureless spectrum and multicomponent T1 were observed below 2 K, indicating that a static magnetic state with incommensurate magnetic correlations or inhomogeneously distributed moments is realized at low temperatures. However, the wide temperature region between 2 K and Tf, where the NQR signal was not observed, suggests that the Ni spins do not freeze immediately below Tf, but keep fluctuating down to 2 K with the MHz frequency range. Below 0.5 K, all components of 1/T1 follow a T3 behavior. We also found that 1/T1 and 1/T2 show the same temperature dependence above Tf but different temperature dependence below 0.8 K. These results suggest that the spin dynamics is isotropic above Tf, which is characteristic of the Heisenberg spin system, and becomes anisotropic below 0.8 K. © 2008 The American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{MacLaughlin2008,

title = {Unconventional spin freezing and fluctuations in the frustrated antiferromagnet NiGa2S4},

author = {D E MacLaughlin and Y Nambu and S Nakatsuji and R H Heffner and L Shu and O O Bernal and K Ishida},

doi = {10.1103/PhysRevB.78.220403},

year  = {2008},

date = {2008-01-01},

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

volume = {78},

number = {22},

abstract = {Muon spin rotation (μSR) experiments reveal unconventional spin freezing and dynamics in the two-dimensional (2D) triangular lattice antiferromagnet NiGa2S4. Long-lived disordered Ni-spin freezing (correlation time ≳10-6 s at 2 K) sets in below T f=8.5±0.5 K with a mean-field-like temperature dependence. The observed exponential temperature dependence of the muon spin relaxation above Tf is strong evidence for 2D critical spin fluctuations. Slow Ni-spin fluctuations coexist with quasistatic magnetism at low temperatures but are rapidly suppressed for fields ≳10 mT, in marked contrast with the field-independent specific heat. The μSR and bulk susceptibility data indicate a well-defined 2D phase transition at Tf, below which NiGa2S4 is neither a conventional magnet nor a singlet spin liquid. © 2008 The American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Ihara20083132,

title = {Co NQR measurements under hydrostatic pressure on superconducting cobaltate Nax (H3 O)z CoO2 · y H2 O},

author = {Y Ihara and H Takeya and K Ishida and K Yoshimura and K Takada and T Sasaki and H Sakurai and E Takayama-Muromachi},

doi = {10.1016/j.jpcs.2008.06.108},

year  = {2008},

date = {2008-01-01},

journal = {Journal of Physics and Chemistry of Solids},

volume = {69},

number = {12},

pages = {3132-3135},

abstract = {Co nuclear quadrupole resonance (NQR) measurements were performed on the bilayer hydrate cobaltate compound Nax (H3 O)z CoO2 · y H2 O under hydrostatic pressures to study the pressure effect on the superconducting (SC) and magnetic properties. We found that the hydrostatic pressure increases the NQR frequency νQ and suppresses the SC transition temperature Tc. The relationship between νQ and Tc observed in the present pressure-dependent measurements is different from that observed in sample-dependent NQR measurements. Nuclear spin-lattice relaxation rate 1 / T1 was also measured under several pressures. The results indicate that the magnetic fluctuations are suppressed with applying pressures. The pressure effect on Tc can be understood as the suppression of magnetic fluctuations, which act as the glue to form the cooper pairs. From the pressure effect on asymmetric parameter η of the electric-field gradient, we suggest a possible explanation for the difference between sample dependence and pressure dependence. Crown Copyright © 2008.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Nakai2008,

title = {Spin dynamics in iron-based layered superconductor (La0.87Ca0.13)FePO revealed by P31 and La139 NMR studies},

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

doi = {10.1103/PhysRevLett.101.077006},

year  = {2008},

date = {2008-01-01},

journal = {Physical Review Letters},

volume = {101},

number = {7},

abstract = {We report P31 and La139 NMR studies of (La0.87Ca0.13)FePO, which is a family member of the recently discovered superconductor LaFeAs(O1-xFx). In the normal state, Knight shift and nuclear spin-lattice relaxation rate divided by T (1/T1T) show that a Fermi-liquid state with moderate ferromagnetic fluctuations emerges below 30 K. From 1/T1T of P31 and La139, a quasi-two- dimensional electronic structure is suggested, in which the FeP layer is more conductive than the LaO layer. In the superconducting (SC) state, although a clear Meissner signal was observed, 1/T1T increases below Tc, in contrast to a decrease of 1/T1T due to the opening of a SC gap, suggesting that novel low-energy spin dynamics develop in the SC state. © 2008 The American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Ihara2008b,

title = {Co-NMR measurements on crystalline sample of the bilayere hydrate Na x(H3O)zCoO2·yH2O},

author = {Y Ihara and K Ishida and H Ohta and K Yoshimura},

doi = {10.1143/JPSJ.77.073702},

year  = {2008},

date = {2008-01-01},

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

volume = {77},

number = {7},

abstract = {Co-nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements were performed on a c-axis-aligned crystalline sample of cobaltate superconductor Nax(H3O)zCoO2· yH2O. The sample properties were characterized in detail by using the NMR and NQR measurements, and the sample was confirmed to exhibit a clear superconducting transition at Tc = 4.56 K. The external field direction was controlled by rotating the sample at low temperatures. The detailed analyses of the NMR spectra revealed that the c-axis direction of the crystalline sample was distributed within ±2.5°. The relaxation rate divided by temperature 1/T1T and Knight shift were measured in the superconducting state. The Knight shift decreased by approximately 0.015% below Tc, which is almost one order smaller than those reported previously. The value of the decrease was the same order as the superconducting diamagnetic effect. The quantitative discussion is given to the Knight shift in the superconducting state. ©2008 The Physical Society of Japan.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Nakai2008b,

title = {Evolution from itinerant antiferromagnet to unconventional superconductor with fluorine doping in LaFeAs(O1-xFx) revealed by 75As and 139La nuclear magnetic resonance},

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

doi = {10.1143/JPSJ.77.073701},

year  = {2008},

date = {2008-01-01},

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

volume = {77},

number = {7},

abstract = {We report experimental results of 75As and 139La nuclear magnetic resonance (NMR) in the iron-based layered LaFeAs(O 1-xFx) (x = 0:0, 0.04, and 0.11). In the undoped LaFeAsO, 1/T1 of 139La exhibits a distinct peak at TN ∼ 142 K below which the spectra become broadened due to the internal magnetic field attributed to an antiferromagnetic (AFM) ordering. In the 4% F-doped sample, 1/T1T exhibits a Curie-Weiss temperature dependence down to 30 K, suggesting the development of AFM spin fluctuations with decreasing temperature. In the 11% F-doped sample, in contrast, pseudogap behavior is observed in 1/T1T both at the 75As and 139La site with a gap value of ΔPG ∼ 172 K. The spin dynamics vary markedly with F doping, which is ascribed to the Fermi-surface structure. As for the superconducting properties for the 4 and 11% F-doped samples, 1/T1 in both compounds does not exhibit a coherence peak just below Tc and follows a T3 dependence at low temperatures, which suggests unconventional superconductivity with line-nodes. We discuss similarities and differences between LaFeAs(O1-xF x) and cuprates, and also discuss the relationship between spin dynamics and superconductivity on the basis of F doping dependence of T c and 1/T1. ©2008 The Physical Society of Japan.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Satoh2008,

title = {Fermi-liquid behavior and weakly anisotropic superconductivity in the electron-doped cuprate Sr1-x Lax CuO2},

author = {K H Satoh and S Takeshita and A Koda and R Kadono and K Ishida and S Pyon and T Sasagawa and H Takagi},

doi = {10.1103/PhysRevB.77.224503},

year  = {2008},

date = {2008-01-01},

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

volume = {77},

number = {22},

abstract = {The microscopic details of flux-line lattice state studied by muon spin rotation are reported in an electron-doped high- Tc cuprate superconductor, Sr1-x Lax CuO2 (SLCO) (x=0.10-0.15). A clear sign of phase separation between magnetic and nonmagnetic phases is observed, where the effective magnetic penetration depth [λ≡λ (T,H)] is determined selectively for the latter phase. The extremely small value of λ (0,0) and corresponding large superfluid density (ns λ-2) is consistent with the presence of a large Fermi surface with carrier density of 1+x, which suggests the breakdown of the "doped Mott insulator" even at the "optimal doping" in SLCO. Moreover, a relatively weak anisotropy in the superconducting order parameter is suggested by the field dependence of λ (0,H). These observations strongly suggest that the superconductivity in SLCO is of a different class from hole-doped cuprates. © 2008 The American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Yamamoto2008,

title = {Low-energy spin fluctuations in filled skutterudites YbFe 4Sb12 and LaFe4Sb12 investigated through 121Sb nuclear quadrupole and 139La nuclear magnetic resonance measurements},

author = {A Yamamoto and S Iemura and S Wada and K Ishida and I Shirotani and C Sekine},

doi = {10.1088/0953-8984/20/19/195214},

year  = {2008},

date = {2008-01-01},

journal = {Journal of Physics Condensed Matter},

volume = {20},

number = {19},

abstract = {We have elucidated low-energy spin fluctuations in the new filled skutterudites YbFe4Sb12 and LaFe4Sb 12 synthesized at high pressures, through 121Sb nuclear quadrupole resonance (NQR) and 139La nuclear magnetic resonance (NMR) measurements. The longitudinal spin-lattice relaxation rate 1/T1 of 121Sb in YbFe4Sb12 provides evidence that upon cooling below ∼20K, the compound transforms from the localized 4f electron state of Yb3+ ions to a nonmagnetic heavy Fermi liquid state, originating from the mixing of 4f electrons with conduction electrons. Whereas, the Curie-Weiss type behaviour of the 139La Knight shift and 121Sb- 1/T1 in LaFe4Sb12 indicate that the compound remains in the localized electron state down to 1.4K, it in fact originates from 3d electrons of Fe in [Fe4Sb12 ] anions. In both compounds, the transversal nuclear spin-spin relaxation rate 1/T2 exhibits a clear peak at and K, respectively. The origin of the 1/T2 peak is discussed in terms of the freezing of the thermal vibration of Sb cages or rare-earth ions filled in each Sb cage. By comparing the experimental results of the present study with those previously reported for the compounds synthesized at ambient pressure, it is pointed out that both the strongly correlated electron properties and the thermal vibrations are greatly modified with the increase of rare-earth atom deficiency. © 2008 IOP Publishing Ltd.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Wada2008,

title = {Competition between valence and spin fluctuations in the vicinity of the quantum critical point of the heavy fermion compound YbAuCu4},

author = {S Wada and A Yamamoto and K Ishida and J L Sarrao},

doi = {10.1088/0953-8984/20/17/175201},

year  = {2008},

date = {2008-01-01},

journal = {Journal of Physics Condensed Matter},

volume = {20},

number = {17},

abstract = {The quantum criticality of the Yb-based heavy fermion compound YbAuCu 4 with noninteger valence close to unity has been investigated through low-temperature resistivity, magnetization, and nuclear magnetic resonance measurements in several fixed magnetic fields H. We found that, with increasing H, YbAuCu4 is driven from the originally antiferromagnetically ordered ground state (Néel temperature T N∼0.9K) to a nonmagnetic Fermi liquid one through the field-tuned quantum critical point (QCP) at kOe. The experimental results also provide the first evidence that a crossover valence transition near the magnetic QCP is stabilized with the application of external field, in marked contrast to the destabilization of the first-order valence transition. The T-H phase diagram of YbAuCu4 and the T-X phase diagram established for the YbXCu 4 (X = Pd, Au, Cu, Ag) series indicate that the evolution of valence fluctuations near the magnetic QCP quickly interacts with the critical spin fluctuations. We suggest that the competition between the Kondo temperature and crossover valence transition temperature is central to the long-standing puzzles of localized-itinerant duality and the extent of degeneracy in the crystal electric field ground state. © 2008 IOP Publishing Ltd.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Nakai20081109,

title = {Site-selective NMR study of the vortex state in an s-wave superconductor},

author = {Y Nakai and Y Hayashi and K Ishida and H Sugawara and D Kikuchi and H Sato},

doi = {10.1016/j.physb.2007.10.261},

year  = {2008},

date = {2008-01-01},

journal = {Physica B: Condensed Matter},

volume = {403},

number = {5-9},

pages = {1109-1111},

abstract = {We report the first site-selective NMR study of the vortex state in an s-wave superconductor. In an s-wave superconductor LaRu4P12(Tc = 7.2 K), a symmetric P-NMR spectrum in the normal state turns into an asymmetrically broadened spectrum below Tc. From the magnetic field dependence of the spectrum, this broadening is due to field distribution arising from the vortex lattice. While there is no frequency dependence of the spin-lattice relaxation rate 1 / T1 in the normal state, significant frequency dependence is observed in 1 / T1 in the vortex state, indicating that the local density of states (LDOS) increases with approaching the vortex. This enhancement of the LDOS with approaching the vortex is considered to result from the confined quasiparticle inside the vortex core. © 2007 Elsevier B.V. All rights reserved.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Ihara20081089,

title = {Magnetic ordering in hydrated cobaltate Nax (H3 O)z CoO2 · y H2 O},

author = {Y Ihara and H Takeya and K Ishida and C Michioka and Y Yoshimura and K Takada and T Sasaki and H Sakurai and E Takayama-Muromachi},

doi = {10.1016/j.physb.2007.10.093},

year  = {2008},

date = {2008-01-01},

journal = {Physica B: Condensed Matter},

volume = {403},

number = {5-9},

pages = {1089-1091},

abstract = {We performed Co nuclear quadrupole resonance (NQR) measurements on bilayered hydrate cobaltate Nax (H3 O)z CoO2 · y H2 O. The sample we used demonstrates a magnetic transition at TM ∼ 6 K, which is observed from susceptibility, nuclear spin-lattice relaxation rate, and NQR spectrum measurements. We observed inhomogeneous broadening of the NQR spectrum below TM. In order to interpret the internal field distribution, we numerically solved a nuclear-spin Hamiltonian including the nuclear quadrupole interaction and the Zeeman interaction, and estimated the internal fields from the observed Co-NQR spectrum. It is considered that the internal fields possess weak modulation. The obtained internal-field distribution suggests the occurrence of the spin-density-wave ordering. © 2007 Elsevier B.V. All rights reserved.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Ohta2008,

title = {Ferromagnetic quantum critical fluctuations and anomalous coexistence of ferromagnetism and superconductivity in UCoGe revealed by Co-NMR and NQR studies},

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

doi = {10.1143/JPSJ.77.023707},

year  = {2008},

date = {2008-01-01},

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

volume = {77},

number = {2},

abstract = {Co nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) studies were carried out for the recently discovered UCoGe, in which the ferromagnetic and superconducting (SC) transitions are reported to occur at TCurie ∼ 3 K and TS ∼ 0.8 K [Huy et al.: Phys. Rev. Lett. 99 (2007) 067006], in order to investigate the coexistence of ferromagnetism and superconductivity as well as the normal-state and SC properties from a microscopic point of view. From the nuclear spin-lattice relaxation rate 1/T1 and Knight-shift measurements, we confirm that ferromagnetic fluctuations that possess a quantum critical character are present above TCurie and also the occurrence of a ferromagnetic transition at 2.5 K in our polycrystalline sample. The magnetic fluctuations in the normal state show that UCoGe is an itinerant ferromagnet similar to ZrZn2 and YCo2. The onset SC transition is identified at TS ∼ 0.7 K, below which 1/T1 arising from 30% of the volume fraction starts to decrease due to the opening of the SC gap. This component of 1/T 1, which follows a T3 dependence in the temperature range 0.3- 0.1 K, coexists with the magnetic components of 1/T1 showing a √T dependence below TS. From the NQR measurements in the SC state, we suggest that the self-induced vortex state is realized in UCoGe. ©2008 The Physical Society of Japan.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Nakai2008d,

title = {Low-lying excitations at the rare-earth site due to the rattling motion in the filled skutterudite la Os4 Sb12 revealed by La139 NMR and Sb 121/123 NQR},

author = {Y Nakai and K Ishida and H Sugawara and D Kikuchi and H Sato},

doi = {10.1103/PhysRevB.77.041101},

year  = {2008},

date = {2008-01-01},

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

volume = {77},

number = {4},

abstract = {We report experimental results of nuclear magnetic resonance (NMR) at the La site and nuclear quadrupole resonance (NQR) at the Sb site in the filled skutterudite La Os4 Sb12. We found that the nuclear spin-lattice relaxation rate divided by temperature 1/ T1 T at the La site exhibits a different temperature dependence from that at the Sb site. Although 1/ T1 T at the Sb site is explained by the Korringa mechanism, 1/ T1 T at the La site exhibits a broad maximum around 50 K, showing the presence of an additional contribution at the La site. The additional low-lying excitations observed at the La site can be understood with the relaxation from anharmonic phonons due to the rattling motion of the La atoms. © 2008 The American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Nakai2008232,

title = {Different spin-echo decay rate behavior of LaT4P12 and LaT4Sb12 (T =Fe, Ru, Os) probed by La-NMR},

author = {Y Nakai and K Ishida and H Sugawara and D Kikuchi and H Sato},

doi = {10.1143/JPSJS.77SA.232},

year  = {2008},

date = {2008-01-01},

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

volume = {77},

number = {SUPPL.A},

pages = {232-234},

abstract = {We report experimental results of La-NMR in the filled skutterudite LaT4P12 and LaT4Sb12 (T =Fe, Ru, Os). 1/T2 in LaT4P12 shows monotonic increase with temperature, which can be modeled qualitatively in terms of the nuclear dipolar interaction and the contribution from electron spins. In contrast, a distinct peak of 1/T2 was observed in LaT4Sb12 although no peak was observed in LaT4P12. Considering these observations, we consider the possible origin of the different 1/T 2 behavior in these compounds. ©2008 The Physical Society of Japan.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kornilov2008683,

title = {Existence of two-dimensional closed orbits and delocalized carriers in the spin-ordered state of quasi-one-dimensional (TMTSF)2PF6},

author = {A V Kornilov and V M Pudalov and K Ishida and T Mito and J S Qualls},

doi = {10.1134/S0021364007220146},

year  = {2008},

date = {2008-01-01},

journal = {JETP Letters},

volume = {86},

number = {10},

pages = {683-686},

abstract = {The magnetoresistance in spin-ordered state of quasi-one-dimensional (TMTSF)2PF6 has been studied. Both the frequency and anisotropy of the magnetoresistance oscillations are found to be independent of temperature; this proves the temperature independence of the geometry of two-dimensional closed contours at the Fermi level. At the same time, the oscillation amplitude decreases with the temperature, indicating the depopulation of the closed contours in the T = 0 limit. An explanation for the above seemingly controversial results has been proposed. © 2007 Pleiades Publishing, Ltd.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Nakai2008333,

title = {Evidence of the bound states of the vortex state in an s-wave superconductor proved by NMR measurements},

author = {Y Nakai and Y Hayashi and K Kitagawa and K Ishida and H Sugawara and D Kikuchi and H Sato},

doi = {10.1143/JPSJS.77SA.333},

year  = {2008},

date = {2008-01-01},

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

volume = {77},

number = {SUPPL.A},

pages = {333-335},

abstract = {In the s-wave superconductor LaRu4P12 (T c=7.2 K), a clear Redfield pattern was observed below Tc, which arises from a triangular lattice. We observed distinct frequency dependence of the spin-lattice relaxation rate 1/T1 in the vortex state in contrast to the normal state. In addition, from the T-dependence of 1/T1T measured at different frequencies, 1/T1T at the vortex core is considered to be greater than that in the normal state, indicating that the LDOS in the vortex core region is higher than that in the normal state. This enhancement of the LDOS in the core region can be understood in terms of the bound states inside a vortex core. ©2008 The Physical Society of Japan.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kitagawa2007,

title = {17O-NMR study in the itinerant metamagnetic compound Sr3 Ru2 O7 for the in-plane field direction},

author = {K Kitagawa and K Ishida and R S Perry and Y Maeno},

doi = {10.1016/j.jmmm.2006.10.1135},

year  = {2007},

date = {2007-01-01},

journal = {Journal of Magnetism and Magnetic Materials},

volume = {310},

number = {2 SUPPL. PART 1},

pages = {e217-e218},

abstract = {17O-NMR experiments on the bilayered perovskite ruthenate Sr3 Ru2 O7 are carried out for various applied fields along the RuO2 plane, for which direction the metamagnetic quantum critical behavior is suppressed. The temperature dependence of the relaxation rate 1 / T1 in the vicinity of the metamagnetism exhibit a significant difference from 1 / T1 for the field along c-axis [K. Kitagawa, K. Ishida, R.S. Perry, T. Tayama, T. Sakakibara, Y. Maeno, Phys. Rev. Lett. 95 (2005) 127001]. © 2006 Elsevier B.V. All rights reserved.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Murakawa2007548,

title = {101Ru Knight-shift measurement of Sr2 RuO4 in low fields applied parallel to the RuO2 plane},

author = {H Murakawa and K Ishida and K Kitagawa and Z Q Mao and Y Maeno},

doi = {10.1016/j.jmmm.2006.10.143},

year  = {2007},

date = {2007-01-01},

journal = {Journal of Magnetism and Magnetic Materials},

volume = {310},

number = {2 SUPPL. PART 1},

pages = {548-550},

abstract = {We have measured 101Ru Knight shift (101K) in the spin triplet superconductor Sr2 RuO4 in various magnetic fields applied parallel and perpendicular to the RuO2 plane to determine the spin direction of the Cooper pair. We measured temperature dependence of 101K in 0.15 and 0.055 T parallel to the RuO2 plane using nuclear-quadrupole-resonance (NQR) technique. We found that 101K is invariant with temperature in both fields on passing through the superconducting (SC) transition temperature. These results indicate that the spin susceptibility parallel to the RuO2 plane does not change in the field greater than 0.055 T, suggesting that the SC d-vector is along the c-axis when the magnetic field greater than 0.055 T is applied along to the RuO2 plane. The possible spin state in magnetic fields is discussed on the basis of the Knight-shift measurements so far. © 2006 Elsevier B.V. All rights reserved.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{MacLaughlin20071300,

title = {Slow inhomogeneous spin dynamics in the frustrated antiferromagnet NiGa2 S4},

author = {D E MacLaughlin and R H Heffner and S Nakatsuji and Y Nambu and K Onuma and Y Maeno and K Ishida and O O Bernal and L Shu},

doi = {10.1016/j.jmmm.2006.10.474},

year  = {2007},

date = {2007-01-01},

journal = {Journal of Magnetism and Magnetic Materials},

volume = {310},

number = {2 SUPPL. PART 2},

pages = {1300-1302},

abstract = {Muon spin relaxation experiments in the geometrically frustrated antiferromagnet NiGa2S4 indicate strong slow spin fluctuations at low temperatures. The muon relaxation rate is strongly field dependent, in contrast with the field independence of the gapless excitations indicated by the observed specific heat. © 2006 Elsevier B.V. All rights reserved.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Murakawa2007b,

title = {101Ru knight shift measurement of superconducting Sr 2RuO4 under small magnetic fields parallel to the RuO 2 plane},

author = {H Murakawa and K Ishida and K Kitagawa and H Ikeda and Z Q Mao and Y Maeno},

doi = {10.1143/JPSJ.76.024716},

year  = {2007},

date = {2007-01-01},

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

volume = {76},

number = {2},

abstract = {We measured 101Ru Knight shift (101K) in Sr 2RuO4 superconductor under low magnetic fields parallel to the RuO2 plane (ab direction) in order to determine the direction of superconducting d vector in zero magnetic field. Kab was measured in a small field down to 550 Oe and in a temperature range down to 97 mK, by using nuclear-quadrupole-resonance (NQR) spectra. We found that Kab is invariant with respect to temperature on passing through Tc. Our results indicate that the spins of the superconducting pair are in the RuO 2 plane (with the superconducting d vector pointing in the c direction) when fields greater than 550 Oe are applied to the RuO2 plane. Taking into account the previous results that 101K is unchanged in the superconducting state when fields greater than 200 Oe are applied along the c axis. We suggest that the effect to fix the spins of the superconducting pairs in a certain crystal direction is weak in Sr 2RuO4 even if there exists a significant spin-orbit interaction originating from the Ru-4d orbitals and additional interactions between the chiral Cooper-pair electrons. ©2007 The Physical Society of Japan.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kitagawa2007b,

title = {Orbital properties of Sr3 Ru2 O7 and related ruthenates probed by O17 NMR},

author = {K Kitagawa and K Ishida and R S Perry and H Murakawa and K Yoshimura and Y Maeno},

doi = {10.1103/PhysRevB.75.024421},

year  = {2007},

date = {2007-01-01},

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

volume = {75},

number = {2},

abstract = {We report a site-separated O17 NMR study of the layered perovskite ruthenate Sr3 Ru2 O7, which exhibits nearly two-dimensional transport properties and itinerant metamagnetism at low temperatures. The local hole occupancies and the spin densities in the oxygen 2p orbitals are obtained by means of tight-binding analyses of electric field gradients and anisotropic Knight shifts. These quantities are compared with two other layered perovskite ruthenates: the two-dimensional paramagnet Sr2 Ru O4 and the three-dimensional ferromagnet SrRu O3. The hole occupancies at the oxygen sites are very large, about one hole per ruthenium atom. This is due to the strong covalent character of the Ru-O bonding in this compound. The magnitude of the hole occupancy might be related to the rotation or tilt of the Ru O6 octahedra. The spin densities at the oxygen sites are also large, 20%-40% of the bulk susceptibilities, but in contrast to the hole occupancies, the spin densities strongly depend on the dimensionality. This result suggests that the density of states at the oxygen sites plays an essential role for the understanding of the complex magnetism found in the layered perovskite ruthenates. © 2007 The American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Ihara20072119,

title = {Unconventional superconductivity near quantum critical point revealed by Co-NQR measurements on Nax (H3 O)z CoO2 · y H2 O},

author = {Y Ihara and H Takeya and K Ishida and C Michioka and K Yoshimura and K Takada and T Sasaki and H Sakurai and E Takayama-Muromachi},

doi = {10.1016/j.jpcs.2007.08.036},

year  = {2007},

date = {2007-01-01},

journal = {Journal of Physics and Chemistry of Solids},

volume = {68},

number = {11},

pages = {2119-2123},

abstract = {The nuclear quadrupole resonance (NQR) frequency of Co nuclei and the nuclear spin-lattice relaxation rate 1 / T1 were measured on several bilayered hydrate (BLH) Nax (H3 O)z CoO2 · y H2 O (y ∼ 1.3) with variety of superconducting (SC) and magnetic transition temperatures, Tc and TM, together with non-SC mono-layered hydrate (MLH) Nax (H3 O)z CoO2 · y H2 O (y ∼ 0.7). In the high temperature region above 70 K, 1 / T1 T in all the samples follows the same temperature dependence which is interpreted as the pseudogap behavior. In the BLH compounds, 1 / T1 T increases with decreasing temperature below 70 K, and the values of 1 / T1 T at Tc are large in high-Tc samples. The magnetic ordering is ascertained from the observation of the prominent divergence of 1 / T1 T at TM in the samples whose NQR frequency is higher than 12.5 MHz. The temperature dependence of 1 / T1 T is found to be consistently expressed by a unique function with two fitting parameters. We analyze the temperature dependence of 1 / T1 T on the basis of this function, and investigate the relationship between the magnetic fluctuations and superconductivity in the BLH compounds. © 2007 Elsevier Ltd. All rights reserved.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Ishida2007190,

title = {Relationship between magnetic fluctuations and superconductivity in Nax(H3O)zCoO2 · yH2O revealed by a Co nuclear quadrupole resonance},

author = {K Ishida and Y Ihara and H Takeya and C Michioka and K Yoshimura and K Takada and T Sasaki and H Sakurai and E Takayama-Muromachi},

doi = {10.1016/j.physc.2007.03.258},

year  = {2007},

date = {2007-01-01},

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

volume = {460-462 I},

number = {SPEC. ISS.},

pages = {190-193},

abstract = {Nuclear quadrupole frequency (νQ) and spin-lattice relaxation rate 1/T1 of Co were measured on various bilayered hydrate (BLH) NaxCoO2 · yH2O (y ∼ 1.3) with different values of superconducting (SC) and magnetic-transition temperatures, Tc and TM, together with non-SC monolayered hydrate (MLH) NaxCoO2 · yH2O (y ∼ 0.7). In the temperature range above 70 K, 1/T1T in all samples follows the similar temperature dependence as each other. In the BLH compounds, 1/T1T is enhanced with decreasing temperature below 70 K, and the values of 1/T1T at Tc are larger in the higher-ν3 sample, where ν3 is the peak frequency of the NQR spectrum arising from ±5/2 ↔ ±7/2 transitions. The magnetic ordering is indicated from the prominent divergence of 1/T1T at TM and the presence of the internal field at the Co nuclear site in the samples with ν3 higher than 12.5 MHz. We analyze the temperature dependence of 1/T1T in all samples on the basis of identical formula, and discuss the relationship between magnetic fluctuations and superconductivity in the BLH cobaltate. © 2007 Elsevier B.V. All rights reserved.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Ihara2007489,

title = {Quantum critical fluctuations in cobaltate superconductor Nax(H3O)zCoO2 · yH2O},

author = {Y Ihara and H Takeya and K Ishida and C Michioka and K Yoshimura and K Takada and T Sasaki and H Sakurai and E Takayama-Muromachi},

doi = {10.1016/j.physc.2007.03.064},

year  = {2007},

date = {2007-01-01},

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

volume = {460-462 I},

number = {SPEC. ISS.},

pages = {489-490},

abstract = {In cobaltate superconductor Nax(H3O)zCoO2 · yH2O with bilayered hydrate (BLH) structure, Co nuclear magnetic resonance (NMR) measurement was performed to investigate the low-temperature magnetic excitations in magnetic field. The nuclear spin-lattice relaxation rate (1/T1) of Co was measured in the field range of 1.5-14 T parallel to the CoO2 plane. In the normal state, 1/T1T follows the temperature dependence expressed as 1 / T1 T ∝ 1 / sqrt(T - θ) in all the field range. At the field lower than 10 T, 1/T1T continues to increase down to Tc and decrease due to superconductivity below Tc. T1T = Const. behavior is not observed at all above Tc. When the field is higher than 10 T, 1/T1T does not show clear decrease, but stop increasing below T*. Using Tc and T* we determine, a H-T phase diagram is developed. The field dependence of Tc at low fields is as small as dTc/dH ∼ -0.06 K/T. A novel phase characterized by a small kink of 1/T1T seems to be present at high-field region above 10 T. © 2007 Elsevier B.V. All rights reserved.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Shu2007,

title = {Muon spin relaxation and hyperfine-enhanced Pr141 nuclear spin dynamics in Pr (Os,Ru)4 Sb12 and (Pr,La) Os4 Sb12},

author = {L Shu and D E MacLaughlin and Y Aoki and Y Tunashima and Y Yonezawa and S Sanada and D Kikuchi and H Sato and R H Heffner and W Higemoto and K Ohishi and T U Ito and O O Bernal and A D Hillier and R Kadono and A Koda and K Ishida and H Sugawara and N A Frederick and W M Yuhasz and T A Sayles and T Yanagisawa and M B Maple},

doi = {10.1103/PhysRevB.76.014527},

year  = {2007},

date = {2007-01-01},

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

volume = {76},

number = {1},

abstract = {Zero- and longitudinal-field muon spin relaxation experiments have been carried out in the alloy series Pr (Os1-x Rux) 4 Sb12 and Pr1-y Lay Os4 Sb12 to elucidate the anomalous dynamic muon spin relaxation observed in these materials. The damping rate Λ associated with this relaxation varies with temperature, applied magnetic field, and dopant concentrations x and y in a manner consistent with the "hyperfine enhancement" of Pr141 nuclear spins first discussed by Bleaney [Physica (Utrecht) 69, 317 (1973)]. This mechanism arises from Van Vleck-like admixture of magnetic Pr3+ crystalline-electric-field-split excited states into the nonmagnetic singlet ground state by the nuclear hyperfine coupling, thereby increasing the strengths of spin-spin interactions between Pr141 and muon spins and within the Pr141 spin system. We find qualitative agreement with this scenario and conclude that electronic spin fluctuations are not directly involved in the dynamic muon spin relaxation. © 2007 The American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Ihara2007c,

title = {Quantum critical behavior in superconducting Nax (H3 O)z Co O2 y H2 O observed in a high-field Co NMR experiment},

author = {Y Ihara and H Takeya and K Ishida and C Michioka and K Yoshimura and K Takada and T Sasaki and H Sakurai and E Takayama-Muromachi},

doi = {10.1103/PhysRevB.75.212506},

year  = {2007},

date = {2007-01-01},

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

volume = {75},

number = {21},

abstract = {Co nuclear magnetic resonance (NMR) measurements are performed on superconducting Nax (H3 O)z Co O2 y H2 O with a bilayered hydrate structure. External fields are applied exactly parallel to the Co O2 plane on a high-quality powdered sample, which is aligned in a strong magnetic field. When superconductivity is suppressed by external magnetic fields, the nuclear spin-lattice relaxation rate divided by temperature (1 T1 T) continues to increase down to Tc (H), which is the superconducting transition temperature under a magnetic field H. From the analyses of the temperature variation, it was found that a sample with Tc ∼4.8 K, which is the maximum Tc, possesses magnetic fluctuations with a quantum critical character. This suggests the intimate relationship between superconductivity and quantum critical fluctuations in the hydrate cobaltate. We determined Tc (H) and constructed the Hc2 - Tc phase diagram from the present NMR measurements. The field dependence of Tc (H) is discussed. © 2007 The American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Nakai2007255,

title = {Rattling phonons in the filled skutterudite LaT4 X12 (T = Fe, Ru, Os ; X = P, Sb) studied with La-NMR, P-NMR/Sb-NQR},

author = {Y Nakai and K Ishida and K Magishi and H Sugawara and D Kikuchi and H Sato},

doi = {10.1016/j.jmmm.2006.10.035},

year  = {2007},

date = {2007-01-01},

journal = {Journal of Magnetism and Magnetic Materials},

volume = {310},

number = {2 SUPPL. PART 1},

pages = {255-257},

abstract = {We report results of La-NMR and P-NMR/Sb-NQR measurements on the La-based filled skutterudite LaT4 X12 (T = Fe, Ru, Os ; X = P, Sb). The temperature dependence of 1 / T1 at the La site is different from that at the X site in LaOs4 Sb12, LaFe4 Sb12, and LaOs4 P12, although no site-dependence in 1 / T1 is observed in LaRu4 Sb12, LaRu4 P12, and LaFe4 P12. We consider that rattling phonon contributes to additional 1 / T1 at the La site, resulting in that a temperature dependence of 1 / T1 at the La site differs from that at the X site. © 2006 Elsevier B.V. All rights reserved.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Ishida2007526,

title = {Magnetic correlations and superconductivity in La1.93 Sr0.07 CuO4 revealed by μSR},

author = {K Ishida and W Higemoto and K Ohishi and A Koda and K H Satoh and R Kadono and M Fujita and K Yamada},

doi = {10.1016/j.jmmm.2006.10.096},

year  = {2007},

date = {2007-01-01},

journal = {Journal of Magnetism and Magnetic Materials},

volume = {310},

number = {2 SUPPL. PART 1},

pages = {526-528},

abstract = {We have performed transverse field muon spin rotation experiments on a single-crystal La1.93Sr0.07CuO4, which is close to the critical value x = 0.06, in the field range from 1 to 60 kOe in order to investigate the relation between magnetism and superconductivity in an underdoped high-Tc superconductor. The measured sample shows evidence of static magnetism at Tg ∼ 4 K, far below the superconducting transition temperature Tc = 17 K. We measured the shift and the linewidth of the fast-Fourier transform (FFT) spectra obtained in a high field of 60 kOe, both of which are dominated by Cu - 3 d spins. The temperature dependences of the shift and the linewidth indicate the development of modulated staggered correlations below 100 K. We conclude that the static staggered correlations coexist with superconductivity far below Tc. © 2006 Elsevier B.V. All rights reserved.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}