水戸 毅

© 2015 American Physical Society. EuPtP undergoes two valence transitions and has two kinds of valence states of Eu ions at low temperatures. In the charge-ordered state, this compound shows an antiferromagnetic order ascribed to magnetic divalent Eu ions. We investigated the antiferromagnetically ordered state of EuPtP by nuclear magnetic resonance (NMR) measurement and nuclear quadrupole resonance (NQR) measurement in a zero external magnetic field. The observed Eu153 NMR signals of a magnetic divalent state and Eu151,153 NQR signals of a nonmagnetic trivalent state clearly demonstrate that the spins order in the hexagonal basal plane and the internal magnetic field is not canceled out, even at the Eu3+ layers which are in the middle of magnetic Eu2+ layers. In addition, the observation of 31P and Pt195 NMR spectra allowed us to discuss a possible magnetic structure. We also evaluated the nuclear quadrupole frequencies for both Eu2+ and Eu3+ ion states.

© 2014 American Physical Society. We have studied EuPtP, which undergoes two successive valence transitions at TA∼240 K and TB∼200 K by P31-nuclear magnetic resonance (NMR) measurements. From the analysis of NMR spectra, we obtained plausible ordered structures and Eu valence states in three phases divided by TA and TB. These ordered structures well explain observed inequivalent P sites and the intensity ratio of the NMR spectra arising from these P sites. The results are also in good accordance with mean Eu valence measured by the x-ray absorption spectroscopy. We also discuss Eu 4f states and the origin of the transitions from the measurements of nuclear spin lattice relaxation rate and hyperfine coupling constant.

Former extensive studies of superconductivity in the A3C60 compounds, where A is an alkali metal, have led one to consider that Bardeen-Cooper-Schrieffer electron-phonon pairing prevails in those compounds, though the incidence of electronic Coulomb repulsion has been highly debated. The discovery of two isomeric fulleride compounds Cs3C60 which exhibit a transition with pressure from a Mott insulator (MI) to a superconducting (SC) state clearly reopens that question. Using pressure (p) as a single control parameter of the C60 balls lattice spacing, one can now study the progressive evolution of the SC properties when the electronic correlations are increased towards the critical pressure pc of the Mott transition. We have used C13 and Cs133 NMR measurements on the cubic phase A15-Cs3C60 just above pc=5.0(3) kbar, where the SC transition temperature Tc displays a dome shape with decreasing cell volume. From the T dependence below Tc of the nuclear spin lattice relaxation rate (T1)-1 we determine the electronic excitations in the SC state, that is 2Δ, the gap value. The latter is found to be largely enhanced with respect to the Bardeen-Cooper-Schrieffer value established in the case of dense A3C60 compounds. It even increases slightly with decreasing p towards pc, where Tc decreases on the SC dome, so that 2Δ/kBTc increases regularly upon approaching the Mott transition. These results bring clear evidence that the increasing correlations near the Mott transition are not significantly detrimental to superconductivity. They rather suggest that repulsive electron interactions might even reinforce elecron-phonon superconductivity, being then partly responsible for the large Tc values, as proposed by theoretical models taking the electronic correlations as a key ingredient. © 2014 American Physical Society.

51V-NMR experiments have been performed to investigate the local magnetic and electronic properties on the phase transition in the mixed-valence oxide δ-Ag2/3V2O5. We have observed the abrupt loss of 51V-NMR signal coming from the non-magnetic V5+-like ions above Tc 225 K. It indicates that the majority of V ions are magnetic above Tc and that the phase transition is accompanied by the charge separation and charge ordering of 3d electrons on V sites. In the charge ordered state below Tc, the magnetic V4+-like ions are located at both V1 and V2 sites with equal distribution.

We report the result of the 33S nuclear magnetic resonance (NMR) measurement on the nonmagnetic semiconductor SmS at ambient pressure. For this measurement, the 33S isotope enriched powder sample of SmS was prepared to increase the 33S NMR intensity. We have attempted 33S NMR measurement on SmS and successfully observed the signal of it. With decreasing temperature, the spectrum measured at the constant magnetic field shifted to lower frequency and became weakly temperature dependent below 50 K. The presence of the energy gap was microscopically established by the rapid decrease in the nuclear spin-lattice relaxation rate 1/T1. The activation energy was deduced to be 625 K from an Arrhenius plot of T1.

The local symmetry at the Ru(4d) and Si(4e) sites in URu2Si 2 was investigated above and below the hidden-order temperature To = 17.5K by measuring 101Ru-nuclear quadrupole resonance (NQR) and 99Ru-nuclear magnetic resonance (NMR) in single-crystalline samples and 73Ge-NMR in a 10%-73Ge- substituted polycrystalline sample. For both sites, no evidence of symmetry change below To was observed. The asymmetry parameter η of the electric field gradient, which reflects the local rotation symmetry, was estimated to be zero with accuracies of 0.0010-0.0015 and 0.05 for the 4d and 4e sites, respectively. The results suggest that the symmetry change at T O is extremely small or intrinsically invisible by the present method. © 2013 The Physical Society of Japan.

We report high-pressure studies of X-ray diffraction and 11B-nuclear magnetic resonance (NMR) in the intermediatevalence compound SmB6. The pressure dependence of the lattice constant was precisely determined and no anomaly was observed up to 9.1 GPa. The temperature dependence of the nuclear quadrupole resonance frequency νQ, obtained from the 11B-NMR measurements, is predominantly contributed by on-site charge distribution. Using the relationship between νQ and Sm valence at ambient pressure reported previously, we estimate the pressure dependence of the Sm valence up to 6 GPa as well. The increase in the Sm valence accelerates with pressure and reaches an increase of about 10% at 6 GPa. The pressure-induced localization of Sm 4f -holes may be responsible for the long-range magnetic order under pressure. © 2013 The Physical Society of Japan.

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

The Yb-based heavy-fermion compound Yb2Ni12P7 with a hexagonal Zr2Fe12P7-type crystal structure was investigated by using the 31P nuclear magnetic resonance (NMR) technique. The complicated NMR line changes its shape gradually with decreasing temperature, implying the presence of some Knight shift components. The temperature dependences of the Knight shift and the nuclear spin-lattice relaxation rate 1/T 1 suggest the delocalization of 4f electrons. © 2013 The Korean Physical Society.

We report the first high-pressure 11B-NMR studies above 3 GPa on the intermediate-valence semiconductor SmB6. A 11B-NMR line obtained at 4.9 GPa, the highest pressure for the measurements, and at 1.9 K shows quite similar a line shape to that at ambient pressure, indicating no structural or magnetic phase transition up to this pressure. The temperature dependence of the spin lattice relaxation rate 1/T 1 at 4.9 GPa still exhibits an activation-type temperature dependence characteristic of semiconductors, which reveals an obvious decrease in the insulating gap by about 30% compared to the gap at ambient pressure. The present experimental facts of a finite insulator gap and no magnetic order at 4.9 GPa are consistent with recent transport measurements performed under better hydrostatic pressures. © 2013 The Korean Physical Society.

We have investigated the noncentrosymmetric superconductor Mo 3Al2C prepared by spark plasma sintering, in which the amount of impurity phase was considerably reduced. Another phase transition in the normal state was clearly observed in the spark plasma sintered sample as an abrupt change in the 27Al nuclear magnetic resonance (NMR) spectra and an anomaly in the electrical resistivity. In the transition, a partial gap opening on the Fermi surface is inferred from the decrease in the nuclear spin-lattice relaxation rate 1/T1 and the magnetic susceptibility Χ below 200 K. The simultaneous presence of two components in the NMR spectra and thermal hysteresis in Χ at ̃200K indicates the first-order character of the transition. These results imply the formation of the charge-density wave state in Mo3Al2C. In the superconducting state, 1/T1 shows a peak just below Tc and decreases exponentially with temperature, in good agreement with that is expected for the s-wave superconductor. © 2013 The Physical Society of Japan.

The discovery in 1991 of high temperature superconductivity (SC) in A3C60 compounds, where A is an alkali ion, has been initially ascribed to a BCS mechanism, with a weak incidence of electron correlations. However various experimental evidences taken for compounds with distinct alkali content established the interplay of strong correlations and Jahn Teller distortions of the C60 ball. The importance of electronic correlations even in A3C60 has been highlighted by the recent discovery of two expanded fulleride Cs3C60 isomeric phases that are Mott insulators at ambient pressure. Both phases undergo a pressure induced first order Mott transition to SC with a (p, T) phase diagram displaying a dome shaped SC, a common situation encountered nowadays in correlated electron systems. NMR experiments allowed us to establish that the bipartite A15 phase displays Néel order at 47K, while magnetic freezing only occurs at lower temperature in the fcc phase. NMR data do permit us to conclude that well above the critical pressure, the singlet superconductivity found for light alkalis is recovered. However deviations from BCS expectations linked with electronic correlations are found near the Mott transition. So, although SC involves an electron-phonon mechanism, correlations have a significant incidence on the electronic properties, as had been anticipated from DMFT calculations. © Published under licence by IOP Publishing Ltd.

We study the normal state properties of a noncentrosymmetric superconductor Mo3Al2C by means of the nuclear magnetic resonance (NMR) experiment. The change of NMR line shape and the peak of the nuclear spin-echo decay rate at≃ 130 K indicate that the Mo3Al2C exhibits the phase transition prior to the emergence of superconductivity. The reduction in the conduction electron density is inferred from the decrease in the nuclear spin-lattice relaxation rate. By considering the result of the energy band calculation, we discussed the nature of the newly found phase transition. © 2012 The Physical Society of Japan.

We have carried out 59Co-nuclear magnetic resonance (NMR) measurements on the heavy Fermion compound YbCo2Zn20, which shows an extremely large specific heat. After examining site symmetry at the Co position by studying a single crystal sample at different directions of magnetic field, we have analyzed powder patterns obtained over a wide range of field from 0.8 to 7.2 T. The temperature dependent component of the Knight shift, which arises from 4f moments, is largely suppressed with increasing field, resulting in weak temperature dependence at low temperatures. This is a sign of field-induced Fermi liquid states, consistent with previous transport measurements on this compound. © 2012 The Physical Society of Japan.

We have investigated the field dependences of Yb valence as well as of the magnetic properties of the heavy-fermion (HF) compound YbCo 2Zn 20 by X-ray absorption spectroscopy, 59Co nuclear magnetic resonance, and magnetization measurements. Our results clearly show that the mechanism of field-induced Fermi-liquid-like behavior does not involve variation in the Yb electronic configuration, and is of magnetic origin. Typical examples of the susceptibility for several Yb-based HF materials are presented, in which one can see systematic changes between localized and delocalized pictures of 4f electrons. © 2012 The Physical Society of Japan.

Quasi-one dimensional organic conductor (TMTSF)2X with octahedral anions X = PF6, AsF6, and SbF6 have been studied by NMR measurements at the Se-sites which are substantially responsible for conductivity in this system. Close resemblances in the temperature dependence of the spin-lattice relaxation rate 1/T1 and the evolution of internal field in the SDW state were observed in the three measured compounds. The existence of the sub-phases in the SDW phase of all these compounds, which is evidenced by T1 but not from spectrum measurements, indicates that this is a common feature among (TMTSF)2X system with an incommensurate SDW wave number and is associated with dynamic property rather than static one.

In order to study local symmetries at the 4e(Si)- and 4d(Ru)-sites above and below hidden order transition temperature TO 17.5 K in URu2Si2, we have investigated the nuclear quadrupole interaction by 73Ge-nuclear magnetic resonance (NMR) measurement on a 10% 73Ge-substituted sample URu2(Si0.9Ge0.1)2, and by 101Ru-nuclear quadrupole resonance (NQR) measurement on a pure single crystal sample. The present 73Ge-NMR measurements do not give any evidence for change in the local symmetry at 4e-site within experimental accuracy. On the other hand, the precise measurement of 101Ru-NQR frequency has detected an anomaly just below TO as sensitively as the thermal expansion measurement dose. Its temperature dependence shows a linear relation with that of the in-plane lattice parameter, which may be a clue to clarify a modification in microscopic charge distribution at TO. © Published under licence by IOP Publishing Ltd.

We report the results of 27Al nuclear magnetic resonance and specific-heat measurements of the noncentrosymmetric superconductor Mo 3Al 2C in the normal state. The phase transition at ∼130 K is inferred from a distinct change in the NMR spectrum, a clear peak in the spin-echo decay rate, and an anomaly in the specific heat. At ∼130 K, the Knight shift and the spin-lattice relaxation rate 1/T 1 exhibit a significant decrease, which is associated with a reduction in the conduction electron density. These results suggest most probably charge-density wave ordering at ∼130 K. Furthermore, the decrease in 1/T 1 below the superconducting transition temperature suggests that superconductivity emerges on the partially quenched Fermi surface. © 2011 American Physical Society.

We have performed measurements of nuclear spin-lattice relaxation rate 1/T1 by 11B-NMR in Kondo insulator system SmB6 under hydrostatic pressure up to 25.5 kbar. 1/T1 increases with pressure and the values of the gap were estimated by fitting the temperature dependence of 1/T1 at different pressures. The obtained pressure dependence of the gap indicates a monotonous but more rapid suppression of the insulating state with pressure than the previous report by the transport measurements. © 2011 The Physical Society of Japan.

The 121/123Sb and 195Pt nuclear magnetic resonance (NMR) results are presented for the half-Heusler compound YbPtSb. The characteristics of localized 4 f spins were observed in the Cure-Weiss type behavior of Knight shifts. The spin-lattice relaxation rate (1/T1) gradually increases with decreasing temperature and shows a maximum at around 20 K. An approximate analysis of the 4f spins correlation rate 1/τf was performed using the measured values of 1/T1 and bulk susceptibility. We observed a √T dependence of 1/τf between 10 and 220 K, which is indicative of an existence of isolated local moment fluctuations. The spin-echo decay rate exhibits a distinct peak at 10 K. We suppose this peak is associated with the magnetic fluctuation originating from 4 f electrons. The crystalline defects in the half-Heusler compounds are also discussed together with the NMR results in LuPtSb. © 2011 The Physical Society of Japan.

In order to investigate the local symmetry at the Si(4e)-site above and below hidden order temperature TO = 17.5 K in URu2Si 2, we have carried out 73Ge-nuclear magnetic resonance (NMR) measurement on 10% 73Ge-substituted sample URu 2(Si0.9Ge0.1)2. The specific heat measurement on the present sample shows a clear second-order transition, indicating that the hidden order is still a bulk property. Besides, equally split 73Ge-NMR lines due to the nuclear quadrupole interaction gives evidence for fairly uniform electric-field gradient at the Ge-substituted sites, Although this is the first attempt to investigate the local symmetry at the 4e-site by proving electric interactions, no evidence for symmetry lowering below TO has been obtained at present. © 2011 The Physical Society of Japan.

121Sb and 19BPt nuclear magnetic resonance (NMR) has been studied in the half-Heusler type compound YbPtSb to obtain information on local magnetic behavior. The characteristics of the localized 4f spins are observed in the Cuire-Weiss type behavior of the Knight shifts K for both 121Sb and 19BPt. From the slope of K-χ plots we estimated hyperfine coupling constants of -3.8 and -4.6 kOe/μB at Sb and Pt sites, respectively. It was found that the spin-echo decay rate 1/T2 of 121Sb shows a clear peaks at 10 K. Similar tendency was also observed in case of 19BPt. However, static properties do not show any anomalies near 10 K. © Published under licence by IOP Publishing Ltd.

We present 103Rh NMR studies for the superconductor Rh 17S15 (Tc = 5.4 K). We have identified the observed NMR lines corresponding to four different Rh sites in the cubic unit cell and deduced the temperature (T) dependence of the Knight shift components in Rh 24m site whose point symmetry is not axial. The isotropic part of the Knight shift K decreases with T in the normal state, indicating the negative hyperfine coupling and the enhancement of the spin susceptibility at lower T. The sudden change of K below Tc is an indication of the spin-singlet Cooper paring. © 2009 Elsevier B.V. All rights reserved.

77Se NMR measurements have been carried out on (TMTSF) 2X (X = PF6 and AsF6) single crystals. For both compounds, NMR lines split into double-peaked spectra in the SDW state, which is explained with sinusoidal internal field at Se nucleus positions having the same incommensurate wave number with that of the SDW order. No change in the lineshape was observed at Tx at which the spin-relaxation rate shows a kink, suggesting that this anomaly does not cause significant static changes in internal field at the Se-site. © 2009 Elsevier B.V. All rights reserved.

Recently magnetic field effects on f electron systems in the vicinity of a magnetic-nonmagnetic transition have attracted much interest. In order to investigate the mechanism of field induced modifications in heavy fermion systems, especially in terms of valent degrees of freedom, we have carried out the X-ray absorption spectroscopy measurement on the newly discovered heavy fermion material YbCo2Zn20 which exhibits extremely large specific heat at low temperatures. Our results show that the Yb valence of YbCo2Zn20 is very close to +3 over the whole temperature range of measurement. Besides, no significant change in the absorption spectrum was observed up to the field of 10 T. This contrasts to the previously reported strong field dependence in the resistivity, followed by an expansion of the Fermi liquid regime with increasing field. The obtained results therefore lead to the conclusion that the valent degrees of freedom play a minor role in the strong field dependence of the heavy fermion systems in this compound.

We present first experimental results of 11B-NMR of SmB 6 under applied pressure. From measurement of nuclear spin-lattice relaxation time (T1) we find that with applied pressure the value of activation gap Eg is decreasing. This decrease is larger than in case of other experimental techniques. We suppose that the enhancement of 1/T 1 in temperature range 20-100 K with applied pressure reflects not only a suppression of hybridization gap, but also changes in spin correlations.

The electronic state of superconductor Rh17S15 has been investigated by Rh-103 nuclear magnetic resonance (NMR) and specific heat measurements. We have identified the observed NMR lines corresponding to four inequivalent Rh sites in the cubic unit cell and obtained the site-selective information of each site. The spin-lattice relaxation rate 1/T-1 indicates that the 4d electrons from the Rh 24m site are the most responsible for the large electronic specific heat coefficient gamma. This conclusion is supported by the energy band calculation. Considering the experimental results and the band calculation, we concluded that the gamma value is enhanced because of the electron correlations of Rh 4d bands. In the superconducting state, an exponential decrease of both 1/T-1 and the electron specific heat are observed, revealing the isotropic superconducting gap with 2 Delta/k(B)T(c) = 4.0. In addition, the decrease of the spin part of the Knight shift indicates the spin-singlet Cooper pair formation. These results demonstrate that Rh17S15 is an s-wave superconductor with enhanced effective electron mass.

103Rh nuclear magnetic resonance (NMR) measurements have been performed in the superconductor Rh17S15 with the transition temperature TC=5.4 K. The observed 103Rh-NMR spectrum shows an asymmetric shape with several peaks, reflecting the local symmetry around each Rh site. We have identified the observed NMR lines corresponding to four different Rh sites and obtained the temperature (T) dependence of the Knight shift of 24m site. The isotropic part of the Knight shift Kiso decreases with decreasing T, indicating the existence of the electron correlation in Rh17S15. In the superconducting state, the resonance lines shift to higher frequencies owing to a decrease of the spin part of the Knight shift with negative hyperfine coupling. © 2010 IOP Publishing Ltd.

The phase transition at T* = 80 K in YbAl3C3 has been studied by 27Al nuclear quadrupole resonance (NQR) measurement and nuclear magnetic resonance (NMR) measurement up to 30 T. The NQR and NMR spectra reveal clear anomalies just below T*, which provides the first microscopic evidence for symmetry lowering in the low-temperature phase below T*. The transition is characterized by marked changes in the 27Al-NQR frequencies νQ's without significant changes in the lattice parameters. The field dependence of the transition is also investigated. The NMR line exhibits field-induced rapid broadening below T*, which almost vanishes in zero field. Moreover, we found that T* increases by 10 K at 30 T. These unique magnetic properties are consistent with the scheme that the microscopic mechanism underlying the phase transition involves Yb-4f electrons rather than the scheme of a simple structural phase transition. ©2009 The Physical Society of Japan.

We report the results of P 31 -nuclear magnetic resonance, R 101 u -nuclear quadrupole resonance (NQR), and ac susceptibility (χac) measurements on NdRu4 P12. This compound is one of the filled skutterudite family R Ru4 P12 (R=rare earth) in which intimate relationships between 4f and conduction electrons with the nesting property of the Fermi surface may play an important role in a variety of intriguing low-temperature states. The NQR and χac measurements indicate the appearance of ferromagnetic ordering below TC =1.7 K. In the paramagnetic state, the coexistence of ferromagnetic and antiferro-type spin correlations are suggested from the field and temperature dependences of the Knight shift and the spin-lattice relaxation time. Such dual spin fluctuations can be explained by considering contributions from both localized 4f and conduction electrons with the nesting condition. © 2008 The American Physical Society.

We report the results of P-31-nuclear magnetic resonance, Ru-101-nuclear quadrupole resonance (NQR), and ac susceptibility (chi(ac)) measurements on NdRu4P12. This compound is one of the filled skutterudite family RRu4P12 (R = rare earth) in which intimate relationships between 4f and conduction electrons with the nesting property of the Fermi surface may play an important role in a variety of intriguing low-temperature states. The NQR and chi(ac) measurements indicate the appearance of ferromagnetic ordering below T-C = 1.7 K. In the paramagnetic state, the coexistence of ferromagnetic and antiferro-type spin correlations are suggested from the field and temperature dependences of the Knight shift and the spin-lattice relaxation time. Such dual spin fluctuations can be explained by considering contributions from both localized 4f and conduction electrons with the nesting condition.

We report nuclear quadrupole resonance (NQR) studies on Ru-sites of filled skutterudite compounds R Ru4 P12 (R = La, Nd, Sm) in zero magnetic field. For NdRu4 P12, the 101Ru-NQR signal disappears below TC = 1.6 K, which is indicative of the transformation into the ferromagnetically ordered ground state. For SmRu4 P12, a single 101Ru-NQR line splits into two lines below the metal-insulator transition temperature TMI = 16.5 K with the intensity ratio of 3 : 1. This result gives evidence that there are two inequivalent Ru-sites with different electric field gradient. The intensity ratio leads to a conclusion that the symmetry is lowered below TMI from a cubic Im over(3, -) space group to a trigonal R over(3, -) one. © 2007 Elsevier B.V. All rights reserved.

We have carried out nuclear quadrupolar resonance (NQR) measurements on SmRu4 P12 and YbAl3 C3, both of which have attracted much interest in terms of multipolar ordering. Although no structural anomalies have been detected by diffraction experiments so far, the NQR resonance frequencies νQ's of both compounds show marked anomalies just below each phase transition temperature, which probably probes the order parameters. The results provide important information to shed light on the ordered structures in the low temperature phases of these compounds. © 2007 Elsevier B.V. All rights reserved.

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.

In order to obtain microscopic information about the metal-insulator (M-I) transition in SmRu4P12, 31P-nuclear magnetic resonance (NMR) measurements using a single crystal have been carried out. Each 31P-NMR line splits into two lines below the M-I transition temperature, consistent with the antiferro-type ordered structure with a wave vector q = (1, 0, 0) proposed by the previous 101Ru-NQR experiments. We also found a remarkable kink in the temperature dependence of the resonance frequencies at 12.5K, which may be related to an anomaly in the temperature dependence of the order parameter. ©2008 The Physical Society of Japan.