Hideyuki Sato

Tokyo Metropolitan University, Edo, Tōkyō, Japan

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Publications (204)323.68 Total impact

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    ABSTRACT: We report the results of Sb nuclear quadrupole resonance (NQR) measurements on the filled skutterudite CeFe4Sb12 synthesized under high pressure (HP) and compare them with those for the sample synthesized under ambient pressure (AP) to understand the influence of the Ce-site filling fraction. The NQR spectra for the HP sample are much sharper than those for the AP sample, suggesting that the Ce filling fraction is higher. Also, the nuclear spin-lattice relaxation rate 1/ T 1 follows an exponential decrease 1/ T 1 ∝ exp(-Δ/ k B T), with the gap being Δ/ k B = 270 K at temperatures above 100 K, which is larger than that for the AP sample. This result suggests that the c- f hybridization is enhanced by increasing the Ce-site filling fraction.
    Journal- Korean Physical Society 08/2013; 63(3):804-808. · 0.51 Impact Factor
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    ABSTRACT: For the unconventional heavy fermion compound SmOs4Sb12, the temperature dependence of the Sm valence state was investigated by Sm L\text{III-edge X-ray absorption spectroscopy under the application of pressure. The Sm valence was found to fluctuate up to a pressure of 3.5 GPa. From the pressure and temperature dependence of the average Sm valence, it was concluded that the Sm divalent contribution plays an important role in the heavy fermion behavior of SmOs4Sb12. Pressure dependence of a characteristic temperature in the Sm valence states exhibits a minimum at around 1 GPa. This finding demonstrates the correlation between the Sm valence and heavy fermion behavior in SmOs4Sb12.
    Journal of the Physical Society of Japan 02/2013; 82(2):3707-. · 2.09 Impact Factor
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    ABSTRACT: We report the results of the 121,123Sb-nuclear quadrupole resonance (NQR) measurements on the filled skutterudite RFe4Sb12 (R = La, Ce) synthesized under high pressure (HP), and compare them with those for the sample synthesized under ambient pressure (AP) in order to understand the effects of the R-site filling fraction on magnetic properties. The NQR spectra for the HP samples are much sharper than those for the AP samples, suggesting that the R-site filling fraction is high close to unity and the electronic states around Sb nuclei become uniform. In LaFe4Sb12-HP, the nuclear spin--lattice relaxation rate divided by temperature, 1/T1T, exhibits the Curie--Weiss temperature dependence 1/T1T=C/(T+θ) with the Weiss temperature θ=12 K above 30 K, indicating the development of ferromagnetic fluctuations. The magnitude of θ for the HP sample is smaller than that for the AP sample, which suggests that the system becomes close to the vicinity of ferromagnetic instability as the La-site filling fraction increases. In CeFe4Sb12-HP, 1/T1 follows the exponential decrease 1/T1 \propto \exp(-Δ/kBT) above 100 K with the gap Δ/kB of 270 K, which is larger than that for the AP sample. This result suggests that c--f hybridization is enhanced by increasing the Ce-site filling fraction.
    Journal of the Physical Society of Japan 12/2012; 81(12):4706-. · 2.09 Impact Factor
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    ABSTRACT: It has recently been found that composite multiplets are formed by the hyperfine coupling between the triplet crystalline-electric-field ground state of the 4f electrons and the 141Pr nuclear spin in Pr ions of the filled skutterudite PrRu4P12. Entropy analysis of the characteristic Schottky-type peak structure appearing in the low-temperature specific heat demonstrates that it is actually caused by thermal excitations of not only 4f electrons but also 141Pr nuclei. The hyperfine coupling constant for Pr ions in nonmetallic PrRu4P12 is determined to be A = +0.050(2) K, which is almost the same as in metallic Pr-based compounds. This fact corroborates that the intra-ion hyperfine coupling of Pr ions is independent of the type of material.
    Journal of Physics Conference Series 12/2012; 391(1):2003-.
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    ABSTRACT: Ac susceptibility (χac) measurements on Pr(Os1-xRux)4Sb12 single crystals for x = 0.05 and 0.1 were performed by mutual inductance method. A peak structure caused by anomalously enhanced flux pinning force, which is so-called the peak effect, appears in the H dependence of χac. The peak structure shifts to lower fields as T increases and disappears in T > 1 K in both samples, while in PrOs4Sb12 it is observable up to near Tc. This fact indicates that the Ru substitution suppresses the peak effect. We demonstrate that the observed T dependent behavior of the peak structure can be explained roughly by the synchronization model although there remains deviation from the model curve, suggesting some modification may be needed in the model to be applied to Pr(Os1-xRux)4Sb12.
    Journal of Physics Conference Series 12/2012; 391(1):2046-.
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    ABSTRACT: The atomic dynamics of the guest modes in heavy fermion filled skutterudites ROs4Sb12 were investigated by inelastic x-ray scattering. Comparison of the phonon dispersion relations for the filled skutterudite SmOs4Sb12 with those for the unfilled skutterudite IrSb3 indicates that the guest modes in filled skutterudites are observed as additional modes in the unfilled skutterudites. The dispersion relations for IrSb3 and SmOs4Sb12 agree reasonably well with the results obtained by ab initio calculations for IrSb3 and LaOs4Sb12, respectively. The rare-earth dependence of the inelastic x-ray scattering spectrum in the series of ROs4Sb12 compounds is difficult to interpret in terms of changes in the atomic mass and electron number of the rare-earth atom inserted into the Sb cage. The combination of inelastic x-ray scattering and x-ray absorption spectroscopy reveals a correlation between the guest mode energy and the guest free space in these compounds. The rare-earth dependence of the inelastic x-ray scattering spectrum indicates the mediation of heavy fermion behavior by the atomic motion of the guest in some ROs4Sb12 compounds and the electron-phonon coupling between the guest modes with 4f-electrons and conduction electrons in the series of ROs4Sb12 compounds.
    Physical review. B, Condensed matter 11/2012; 86(19). · 3.77 Impact Factor
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    ABSTRACT: We report inelastic neutron scattering results for single-crystalline samples of PrFe4P12, which undergoes a transition from the heavy-electron phase to the antiferro-type nonmagnetic ordered phase. In contrast to the quasielastic magnetic response observed in the heavy electron phase, the well-defined inelastic peaks located at 1.5 and 3 meV are superimposed in the ordered phase. The maximum spectral intensity appears at the Brillouin zone center for the original Im\bar{3} structure, and the minimum spectral intensity appears at the zone boundaries including the ordering vector \mbi{q}0=(1,0,0). The evolution of the 1.5 meV peak with an applied magnetic field in the ordered phase is in agreement with excitations between 4f-electron crystalline-field-splitting levels, while the 3 meV peak is smeared out rapidly. The spectra are consistent with the recently proposed staggered ordering of a Pr site with well-localized f-electrons and that with a Kondo-singlet-like state, which are represented as the totally symmetric state accompanied by the charge modulation of carriers.
    Journal of the Physical Society of Japan 09/2012; 81(9):4711-. · 2.09 Impact Factor
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    ABSTRACT: We have investigated the electrical resistivity ρ and Hall resistivity ρH of the filled skutterudite PrFe4P12 as functions of both temperature T down to 30 mK and magnetic field H up to 17.9 T for the field directions near the [111] axis. At low temperatures, we observed a clear anomaly in ρ(H) and ρH(H) within a limited range of field directions close to H\parallel [111] at around 8 T, reflecting the onset of the high-field ordered phase (HOP) reported previously. ρ exhibits a crater like structure as a function of the field direction in the (\bar{1}10) plane with a minimum near the [111] axis. Magnetoresistance exhibits clear anomalies across the boundaries of both the low-field scalar-order phase (SOP) and the HOP. With decreasing temperature, the phase boundaries of the high-field side of the SOP and the low-field side of the HOP approach more slowly than was inferred previously. The carrier density in the HOP evaluated by the ρ and ρH measurements is more than an order of magnitude higher than that in the SOP, which rules out \mbi{q}= (0,0,1) as the ordering wave vector for the HOP. Within the HOP, we found an additional anomaly accompanied by a clear hysteresis at around 17 T below ˜0.75 K, indicating the existence of a new phase boundary. The existence of the additional state in the HOP may indicate that almost-degenerate multiple internal degrees of freedom are involved in the HOP. This finding should provide an important clue to clarify the nature and origin of the HOP.
    Journal of the Physical Society of Japan 08/2012; 81(8):4703-. · 2.09 Impact Factor
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    ABSTRACT: We have investigated the temperature and pressure dependency of the electronic structure of Yb-filled skutterudites, YbFe(4)Sb(12) and Yb(0.88)Fe(4)Sb(12), using x-ray absorption and emission spectroscopies. An anomalous increase of the Yb valence, which is beyond the conventional Anderson model picture, is found to coincide with the onset of the ferromagnetic order in the x=0.88 sample below 20 K. In contrast, the nearly stoichiometric YbFe(4)Sb(12) is paramagnetic down to 2 K and the Yb valence is independent of temperature. This evidences a close interplay between the magnetic instability of the Fe 3d electrons and valence instability of the Yb 4f electrons. Under pressure, a sudden increase in the valence is found to occur around 13 GPa for YbFe(4)Sb(12) and 17 GPa for Yb(0.88)YbFe(4)Sb(12).
    Physical Review Letters 10/2011; 107(17):177203. · 7.73 Impact Factor
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    ABSTRACT: In the unconventional f-electron-associated charge order phase of filled skutterudite PrRu4P12, the low-temperature behaviors of the triplet crystalline-electric-field ground state of Pr ions have been studied by specific heat and magnetization measurements using high quality single crystals. Specific heat shows an anomalous Schottky-type peak structure at 0.30 K in zero field in spite of the absence of any symmetry breaking. Magnetization curve at 0.06 K shows a remarkable rounding below 1 T. It has been revealed that these anomalies provide compelling evidence for the formation of a lattice of Pr 4f-electron-nuclear hyperfine-coupled multiplets, the first known thermodynamical observation of its kind.
    Journal of the Physical Society of Japan 05/2011; 80. · 2.09 Impact Factor
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    ABSTRACT: The magnetic and transport properties of single crystalline YbT2Al20 (T = Ti, V and Cr) grown by Al self-flux method were investigated. All compounds show weakly-temperature dependent Pauli paramagnetic behavior in the DC susceptibility and show normal metallic behavior without any noticeable shoulders and peaks in the temperature dependence of resistivity. From these measurements, the valence state of Yb ions is expected to be close to divalent.
    Journal of Physics Conference Series 02/2011; 273(1):012033.
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    ABSTRACT: 121,123Sb nuclear magnetic resonance (NMR) measurements of a high-quality single crystalline PrOs4Sb12 have been carried out in magnetic fields up to 98 kOe and in the temperature range of 0.4--30 K. From the analysis of the 121Sb Knight shift (K) and magnetic susceptibility at H≈ 90 kOe, the transferred hyperfine coupling constants (ATHF) are obtained. The Sb isotropic hyperfine coupling constant and anisotropic hyperfine coupling tensor are determined as ≈27.6 Oe/muB and ≈(149.9,42.3,-192.2) Oe/muB, respectively. The anisotropic part originates from the spin--dipolar field due to the local spin density at Sb 5p orbital transferred through the hybridization with 4f electrons. The results are consistent with the Sb12 5p molecular orbital with au symmetry predicted from the band structure calculation. ATHF for H\parallel [001] is dependent on the magnetic field, suggesting the coupling of the Sb nuclear dipole moment with not only dipole moments but also octupole moments of the Pr 4f electrons with the same time reversal. The NMR relaxation rate depends both on magnetic field and temperature, and the temperature dependence of 1/T1 is explained by the phenomenological expression 1/(T1TATHF2)\propto (1/T) \exp[-DeltaNMR(H)/T] with excitation gap DeltaNMR(H). Here, we found that DeltaNMR(H) is proportional to the largest excitation gap between Gamma1 and Gammat- of the triplet Gamma4(2) state under the magnetic field. These results indicate that the crystal electric field (CEF) excitation and the degeneracy of the triplet state are important for understanding Sb-NMR results. It is strongly indicated that the mass enhancement in PrOs4Sb12 is caused by the scattering conduction electrons with multipole fluctuations via the singlet--triplet CEF excitation.
    Journal of The Physical Society of Japan - J PHYS SOC JPN. 01/2011; 80.
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    ABSTRACT: YbFe4Sb12 single crystals with the Yb-site filling fraction close to 100% have been successfully synthesized by the Sb self-flux method under a pressure of 4 GPa. The grown single crystals do not exhibit ferromagnetic ordering down to the lowest temperature investigated, in contrast with the ferromagnetic transition reported for Yb-deficient single crystals grown at ambient pressure. The electronic specific heat coefficient of the present crystals is evidently smaller than those reported for Yb-deficient samples, indicating that the Fermi energy is located at the high-energy side of a peak in the electronic density of states dominated by the contribution of Fe-3d electrons. Therefore, the appearance of the ferromagnetic state in the Yb-deficient samples can be naively understood within the simple Stoner-type argument. Marked enhancement of the positive magnetoresistance was found in the temperature region where the spin-fluctuation scattering of conduction electrons dominates.
    Journal of The Physical Society of Japan - J PHYS SOC JPN. 01/2011; 80.
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    ABSTRACT: Extended x-ray absorption fine structure (EXAFS) measurements were carried out at the K-edge of rare-earth ions in REFe4P12 (RE = La, Ce, Pr, Nd, and Sm) at various temperatures. Distance between a rare-earth atom and neighboring atoms in each REFe4P12 compound was determined in this work. The obtained results agree reasonably with previous results of diffraction experiments. The temperature dependence of EXAFS amplitude was successfully analyzed using the Einstein model in each compound. The Einstein temperatures obtained by the analyses show a significant rare-earth dependence. Quantitative comparison between the present EXAFS measurements and phonon measurements such as inelastic x-ray/neutron scattering suggests that reduced mass treatment is essential for estimating Einstein temperatures. This indicates that finite binding energy exists between a rare-earth atom and P atoms at least in REFe4P12 compounds.
    Journal of The Physical Society of Japan - J PHYS SOC JPN. 01/2011; 80.
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    ABSTRACT: Magnetization, electrical resistivity and specific heat measurements were performed on high-quality single crystalline SmTi2Al20 (residual resistivity ratio ˜ 40) grown by Al self-flux method. A Kondo-like log T dependence in the resistivity is observed below 50 K. We discovered a field-insensitive phase transition at Tx = 6.5 K and a field-insensitive heavy fermion behavior with the electronic specific heat coefficient gamma = 150 mJ/(K2 mol). Specific heat analysis reveals that the ground state is a Gamma8 quartet state and the Sm magnetic dipole moment mSm (˜ 0.5muB at T ~= 0) orders below Tx in spite of the field-insensitive behavior. Possible reasons for the field insensitiveness will be discussed.
    Journal of the Physical Society of Japan 01/2011; 80. · 2.09 Impact Factor
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    ABSTRACT: PrOs4Sb12 is the first Pr-based heavy fermion superconductor exhibiting intriguing behavior both in the superconducting and normal states. To clarify the origin of the behavior, intensive studies have been made by variety of experimental techniques. As a result of intensive studies, the normal state properties have been mostly clarified, however, there still remain important questions on the superconducting state of this material. We discuss the present status of the understanding of superconducting state in relation with the normal state properties of this material.
    Physica C Superconductivity 12/2010; 470. · 0.72 Impact Factor
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    ABSTRACT: By specific heat measurements on Pr(Os1−xRux)4Sb12, we have studied the x-dependences of the 4f-electron crystalline-electric-field first excitation energy and the rattling energy of the Pr ion. The former increases monotonically from ~ 10 K to ~ 85 K, while the latter of ~ 45 K does not change much resulting in a crossing of the two energy levels at x ≊ 0.6. The reported minimum in the superconducting transition temperature Tc at x ≊ 0.6 could be associated with the level crossing.
    Journal of Physics Conference Series 02/2010; 200(1):012125.
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    ABSTRACT: ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
    ChemInform 01/2010; 31(10).
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    ABSTRACT: The order parameter in the pressure-induced insulating phase of the filled skutterudite compound PrFe4P12 has been investigated using a high-pressure single-crystal neutron diffraction technique. Clear evidence of the antiferromagnetic order with the propagation vector q = (1, 0, 0) was observed in the insulating phase above 2.7 GPa. The q = (1, 0, 0) structure is identical with that in the nonmagnetic ordered phase below 2.5 GPa and with the nesting property of the Fermi surface. The observed magnetic moment of the Pr ion is 2 muB and almost pressure-independent up to at least 4.2 GPa. The 2 muB magnetic moment originates from the dipole in the low-lying quasi-quartet crystal field state. The pressure-induced antiferromagnetic structure at 3.2 GPa is suppressed by applying a magnetic field of about 1.5 T, which is interpreted as a flop of magnetic moments into the alignment of the ferromagnetic state. This brings about a steep decrease in the electrical resistivity due to the disappearance of the antiferromagnetic superstructure.
    Journal of the Physical Society of Japan 01/2010; 79(3). · 2.09 Impact Factor
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    ABSTRACT: To investigate the quantum criticality and spin fluctuations in LaFe4As12, we have investigated the pressure and magnetic field dependences of electrical resistivity (rho). The Curie temperature TC (˜ 5.2 K at ambient pressure) is gradually suppressed by pressure (P) and disappears at approximately 2 GPa (˜ Pc). The coefficient A of the T2 dependence of rho increases near Pc, and the exponent n of rho(T)˜rho0+beta Tn changes into 5/3, which is expected for 3d ferromagnetic spin fluctuations. The low-field longitudinal magnetoresistance Deltarho/rho(T) is negative at low temperatures, as was commonly observed in typical itinerant-electron weak ferromagnets, reflecting a field-suppressed spin-fluctuation scattering. Above TC, it becomes positive and shows a peak against magnetic field, which is hardly ascribable only to the ordinary positive magnetoresistance due to the Lorentz force on conduction electrons. Possible origins of the unexpected positive magnetoresistance are discussed by considering Kohler's plot for both the longitudinal and transverse magnetoresistances.
    Journal of the Physical Society of Japan 01/2010; 79(6). · 2.09 Impact Factor