水戸 毅

We present the results of nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements on the layered tin pnictide superconductors NaSn2As2 (the superconducting transition temperature Tc = 1.3 K) and Na1−xSn2P2 (Tc = 2.0 K). In NaSn2As2, a broad 75As NQR spectrum indicates a large charge distribution at the As site, and a broad 119Sn NMR spectrum indicates a distributed local density of states (DOS) at the Sn site. Almost the same level of microscopic disorder in the electronic state is also found at the Sn site in Na1−xSn2P2 as well. These results clearly indicate the presence of a local atomic disorder in these superconductors. We have also measured the nuclear spin–lattice relaxation rate 1=T1 to gain insight into the partial DOS at each site. Our NMR=NQR results are consistent with previous band calculations, indicating that Na deficiency results in an increase in the DOS at the Fermi energy (EF) due to the decrease in EF. The agreement between the experimental and band calculation results makes it more plausible that the increase in the Tc of Na1−xSn2P is due to the increase in the DOS at EF caused by the Na deficiency. This study thus demonstrates that NMR=NQR measurements can be a very effective tool in revealing the local electronic structure of materials by comparing experimental results with theoretical band calculations.

We applied the Bayesian framework to the analysis of nuclear spin-lattice relaxation curves. In the simplest case, nuclear spin-lattice relaxation is described by a single exponential function. However, relaxations to which the exponential function does not fit well are often observed in real NMR experiments. One of the reasons for those relaxations is the sample inhomogeneity. For such cases, it is common practice to analyze the curve by fitting a stretched exponential function exp(-(t=T1)beta) to the data, where T1 is the nuclear spin-lattice relaxation time. Another possibility is the coexistence of different relaxation mechanisms. In this situation, the relaxation curves should be fitted by the summation of several relaxation functions. For scientists to interpret relaxation curves, a data-driven method is required to determine which of the two possibilities is more plausible. In this paper, we propose a Bayesian method to select the appropriate relaxation function and even the number of relaxation components. We demonstrate that our method effectively determines the relaxation model on the basis of numerical experiments and the experimental data of nuclear spin-lattice relaxation of the nonmagnetic semiconductor SmS.

Dirac fermion systems form a unique Landau level at the Fermi level-the so-called zero mode-whose observation itself will provide strong evidence of the presence of Dirac dispersions. Here, we report the study of semimetallic black phosphorus under pressure by ^{31}P-nuclear magnetic resonance measurements in a wide range of magnetic field up to 24.0 T. We have found a field-induced giant enhancement of 1/T_{1}T, where 1/T_{1} is the nuclear spin lattice relaxation rate: 1/T_{1}T at 24.0 T reaches more than 20 times larger than that at 2.0 T. The increase in 1/T_{1}T above 6.5 T is approximately proportional to the squared field, implying a linear relationship between the density of states and the field. We also found that, while 1/T_{1}T at a constant field is independent of temperature in the low-temperature region, it steeply increases with temperature above 100 K. All these phenomena are well explained by considering the effect of Landau quantization on three-dimensional Dirac fermions. The present study demonstrates that 1/T_{1} is an excellent quantity to probe the zero-mode Landau level and to identify the dimensionality of the Dirac fermion system.

The zero-field state in the antiferroquadrupole (AFQ) ordered phase of CeB6 has been investigated by a B-11-nuclear quadrupole resonance (NQR) measurement that is sensitive to changes in local charge distribution. In order to achieve this experiment, we have improved our low-frequency NQR techniques, so that the B-11-NQR spectrum has been successfully observed at an NQR frequency nu(Q) similar to 560 kHz, exceptionally low frequency among direct NQR measurements on strongly correlated electron systems. The temperature dependence of the NQR spectrum reveals that nu(Q) rapidly decreases just below AFQ ordering temperature T-Q = 3.3 K, while the spectral shape does not show any sign of site splitting below T-Q. The latter is incompatible with the theoretical prediction based on the O-xy-type AFQ order, suggesting that the ordered state in the zero-field region differs from that in the nonzero-field region. Some new possible interpretations to explain the obtained results are proposed.

A new ternary layered pnictide, Li1-xSn2+xP2, was synthesized by a solid-state reaction and its properties were examined to explore its potential as a multifunctional material. The compound crystallizes in a layered structure in the R3m space group (no. 166) with buckled honeycomb Sn-P layers separated by mixed-occupation Li/Sn layers. Crystal structure analysis by synchrotron X-ray diffraction showed that the substitution degree of Li by Sn is x = 0.38. The local ordering of Li/Sn occupation was demonstrated using P-31 nuclear magnetic resonance analysis. The thermal and electrical transport properties are significantly affected by this local ordering. The lattice thermal conductivity of Li1-xSn2+xP2 was found to be relatively low (1.2 W m(-1) K-1 at 525 K). The room-temperature electrical resistivity of Li1-xSn2+xP2 was found to be 0.3-0.4 m omega cm and metallic conductivity was observed down to 0.5 K. First-principles calculations demonstrated that the electronic structure and Fermi energy of Li1-xSn2+xP2 are significantly dependent upon x. Moreover, the electronic structure of Li1-xSn2+xP2 is different from that of the related compound NaSn2As2, which shows a superconducting transition. Electrochemical measurements using a single-particle technique demonstrated the activity of Li1-xSn2+xP2 as an anode material for rechargeable Li-ion batteries.

© 2020 American Physical Society. ©2020 American Physical Society. Two-dimensional layered semiconductor black phosphorus (BP), a promising pressure-induced Dirac system as predicted by band structure calculations, has been studied by P31 nuclear magnetic resonance. Band calculations have been also carried out to estimate the density of states D(E). The temperature and pressure dependences of the nuclear spin lattice relaxation rate 1/T1 in the semiconducting phase are well reproduced using the derived D(E), and the resultant pressure dependence of the semiconducting gap is in good accordance with previous reports, giving a good confirmation that the band calculation on BP is fairly reliable. The present analysis of 1/T1 data with complemental theoretical calculations allows us to extract essential information, such as the pressure dependences of D(E) and the chemical potential, as well as to decompose the observed 1/T1 into intrinsic and extrinsic contributions. An abrupt increase in 1/T1 at P=1.63GPa indicates that the semiconducting gap closes, resulting in the enhancement of conductivity.

© 2017 Elsevier B.V. We report the results of the X-ray absorption spectroscopy (XAS) on the intermediate valence compound SmB6. The XAS measurements were performed near the nonmagnetic-magnetic phase boundary. Mean Sm valence vSm was estimated from absorption spectra, and we found that vSm near the boundary (P≥10GPa and T∼12K) is far below a trivalent state with magnetic characteristics. Although the result is markedly different from the cases of pressure induced magnetic orders in Yb and Ce compounds, it is likely that the large deviation from the trivalent state seems to be common in some Sm compounds which possess electronic configuration between 4f5 and 4f6 with multi 4f electrons.

© 2018 American Physical Society. We report a systematic study of Sm valence in the prototypical intermediate valence compound SmB6. Sm mean valence vSm was measured by x-ray absorption spectroscopy as a function of pressure (1<P<13GPa) and temperature (3<T<300K). Pressure-induced magnetic order was detected above Pc=10GPa by resistivity measurements. A shift toward the localized 4f state with increasing P and/or T is evident from an increase in vSm. However vSm at Pc is anomalously far below 3, which differs from the general case of nonmagnetic-magnetic transition in Yb and Ce compounds. From the T dependence of vSm(P,T), we found that vSm(P,T) consists of two different characteristic components: One is associated with low-energy electronic correlations involving Kondo-like behavior, and the other is associated with high-energy valence fluctuations.

© 2017 The Author(s). A pressure-induced anomalous valence crossover without structural phase transition is observed in archetypal cubic YbCu5 based heavy Fermion systems. The Yb valence is found to decrease with increasing pressure, indicating a pressure-induced crossover from a localized 4f 13 state to the valence fluctuation regime, which is not expected for Yb systems with conventional c-f hybridization. This result further highlights the remarkable singularity of the valence behavior in compressed YbCu5-based compounds. The intermetallics Yb2Pd2Sn, which shows two quantum critical points (QCP) under pressure and has been proposed as a potential candidate for a reentrant Yb2+ state at high pressure, was also studied for comparison. In this compound, the Yb valence monotonically increases with pressure, disproving a scenario of a reentrant non-magnetic Yb2+ state at the second QCP.

© 2017 American Physical Society. We have carried out Si29 nuclear magnetic resonance of URu2Si2, which undergoes the so-called hidden order (HO) at THO=17.5 K. The use of a Si29-enriched sample enabled us to obtain accurate NMR data up to 350 K and to extract the features of low energy magnetic correlations at high temperatures in this compound. Present results naturally indicate that URu2Si2 undergoes a crossover from localized to itinerant regimes with decreasing temperature and the HO occurs in the crossover region. We also discuss possible antiferromagnetic correlations remaining down to THO by extracting magnetic fluctuation components.

© Published under licence by IOP Publishing Ltd. The non-f compound ThRu2Si2 has been studied as an isostructural nonmagnetic reference for URu2Si2 using nuclear magnetic resonance (NMR) measurement. The temperature dependences of Knight shifts measured by 29Si-NMR and 99Ru-NMR are independent of temperature. The results are consistent with data previously reported on the susceptibility. 101Ru-NQR frequency 101 ν Q was also estimated from the 99Ru-NMR measurement. 101 ν Q of ThRu2Si2 is close to that of URu2Si2, especially at high temperatures, suggesting that U ions in URu2Si2 are in a nearly tetravalent state.

© Published under licence by IOP Publishing Ltd. We have carried out the high-pressure measurement of X-ray absorption spectroscopy on the intermediate valence compound SmB6 which shows magnetic ordering as well as an insulator-metal transition at critical pressure Pc ∼ 10 GPa. The valence of Sm atom at room temperature increases with increasing pressure, however it is far below a trivalent state at Pc . In contrast to cases of pressure-induced nonmagnetic-magnetic transition in Yb compounds, which mostly occurs in the scheme of well localized 4f electrons, the present observation suggests that electronic system in SmB6 still possesses strong delocalized characters at Pc .

© Published under licence by IOP Publishing Ltd. We have performed 29Si nuclear magnetic resonance experiment to investigate microscopic magnetic properties of EuNi2Si2 with trivalent Eu ion. The temperature dependence of the nuclear spin-lattice relaxation rate 1/T 1 can be explained by considering the two relaxation processes: conduction electrons and the thermally activated spins in the excited levels of J-multiplet. The temperature dependence of the Knight shift could roughly be reproduced with the spin-orbit coupling constant estimated from 1/T 1.

© 2017 American Physical Society. We present a detailed NMR study of the insulator-to-metal transition induced by an applied pressure p in the A15 phase of Cs3C60. We evidence that the insulating antiferromagnetic (AFM) and superconducting (SC) phases coexist only in a narrow p range. At fixed p, in the metallic state above the SC transition Tc, the Cs133 and C13 NMR spin-lattice relaxation data are seemingly governed by a pseudogaplike feature. We prove that this feature, also seen in the Cs133 NMR shift data, is rather a signature of the Mott transition which broadens and smears out progressively for increasing (p,T). The analysis of the variation of the quadrupole splitting νQ of the Cs133 NMR spectrum precludes any cell symmetry change at the Mott transition and only monitors a weak variation of the lattice parameter. These results open an opportunity to consider theoretically the Mott transition in a multiorbital three-dimensional system well beyond its critical point.

? Published under licence by IOP Publishing Ltd. 51V-NMR experiments have been performed to investigate the local magnetic and electronic properties of the mixed-valence oxide beta-Ag2/3V2O5, which shows the novel V4+/V5+charge ordering trigged by Ag ion ordering. We have observed the abrupt loss of51V-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 Tcand that the phase transition is accompanied by the charge separation and the charge ordering of 3d electrons on V sites. In the lowerature phase below Tc, the nuclear spin-lattice relaxation rate 1/T1shows the thermal activation-type temperature dependence with the activation energy of about 130 K. It has been clarified that the ground state of this material is the charge ordered one with the spin singlet of 3d electrons on V4+-V4+pair from a microscopic point of view.

<p>これまで、A15型超伝導体V3Siの常伝導状態および超伝導状態について多くの報告がV-NMRによりなされてきた。しかし、その結果見積もられたバンド幅がバンド計算の結果と合わないなど不明確に残された点があった。そこで今回我々はそれらの問題を明らかにするため、Si-NMRによる測定を行ったのでその測定の結果を報告する。</p>

©2016 The Physical Society of Japan. A diffraction experiment using high-energy X-rays was carried out on YbInCu4. Below the Yb valence transition temperature, the splitting of Bragg peaks was detected in high-order reflections. No superlattice reflections accompanying the valence ordering were found below the transition temperature. These experimental findings indicate that a structural change from a cubic structure to a tetragonal structure without valence ordering occurs at the transition temperature. Such a structural change free from any valence ordering is difficult to understand only in terms of Yb valence degrees of freedom. This means that the structural change may be related to electronic symmetries such as quadrupolar degrees of freedom as well as to the change in Yb valence.

© 2016 American Physical Society. We have carried out the measurements of high-pressure B11-nuclear magnetic resonance on the intermediate-valence compound SmB6 to investigate the effects of pressure on Sm 4f states and the quasiparticle band. From the measurements of spin-lattice relaxation time, just below the critical pressure Pc of nonmagnetic-magnetic phase transition, we find that quasiparticle bandwidth clearly decreases with pressure, while the insulating gap is almost constant or slightly increases. The latter is consistent with the result of a band-structure calculation. These pressure induced modifications in the band structure indicate the enhancement of the density of states of the quasiparticles when approaching Pc. The pressure dependence of the Sm 4f states and the origin of the insulating gap are well explained in terms of exchange interactions between conduction and 4f electrons.

<p>黒リンは層状構造のナローギャップ半導体であるが、圧力印加によってギャップが減少し、グラファイトやビスマスのような半金属物質に転移する。本研究では黒リンの半導体特性を微視的実験手法である核磁気共鳴法を用い調べた。常圧下の核スピン格子緩和率は、半導体特性を反映して高温に伴い減少を示した。講演では、半金属状態への転移とそこでの電子状態を明らかにするため、高圧下での測定結果も報告する予定である。</p>

<p>Lu_5_Ir_4_Si_10_などは、電荷密度波転移（CDW）を示唆する物性を示し、さらに低温でCDWと超伝導が共存している可能性がある。そこで、NMRを用いて各サイトの電子状態を調べ、CDWおよび超伝導の発現機構について考察する。</p>

<p>六法晶をとるSmGa_2_は20Kで反強磁性秩序するが10K以下で磁化率が強磁性的に増大することが報告されている．しかし，基底状態の磁気構造はまだわかっていない．我々は外部磁場を印加せず，磁気秩序による内部磁場によるGaのNMRスペクトルの観測に成功した．この解析により磁気構造を考察する．</p>

<p>SmB_6_は小さな半導体ギャップを持つ近藤半導体であり、また常圧下ではSm価数が約2.6価の中間価数物質である。この物質は臨界圧力7－10GPaで金属化することが知られており、その圧力下での低温では磁気秩序相が出現することが分かっている。我々は、この物質のギャップが圧力下で消失する機構や、磁気秩序相での電子状態を微視的に調べるために、臨界圧力近傍において^11^B-NMR測定を行ったのでその結果を報告する。</p>

<p>SmB_6_は、小さなエネルギーギャップを持つ近藤半導体であり、電気抵抗測定からLa置換による金属化が報告されているが、核磁気共鳴(NMR)測定ではSmサイトに局所的な何らかの励起が存在することがわかっている。本研究では、La:0~90%の粉末試料とLa:57%の単結晶試料について、BとLaサイトの系統的なNMR測定を行った。講演では、ミクロな視点から局所的励起と金属化の機構について議論する。</p>

©2015 The Physical Society of Japan. We report the detailed analysis of the 101Ru nuclear quadrupolar resonance spectrum in the ferromagnetically ordered state of EuRu4Sb121 and propose that Eu 4f moments align in the [111] direction. The localized character of Eu 4f electrons is suggested from the temperature dependence of the nuclear spin-lattice relaxation rate.

©2015 The Physical Society of Japan. We have carried out nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements on ThRu2Si2 and LaRu2Si2, which are the nonmagnetic references of the intriguing heavy fermion URu2Si2. The comparisons of URu2Si2 with the reference materials allow us to analyze the already known NMR and NQR data on URu2Si2 phenomenologically and semiquantitatively. The study of 101Ru-NQR frequency suggests the relatively close electronic configuration of URu2Si2, including the valence of the actinide ion, to that of the tetravalent ThRu2Si2 at high temperatures, as well as the delocalization of 5 f electrons at low temperatures. Ising-like spin fluctuations along the c-axis were brought to light by 29Si-NMR data in the so-called hidden order phase of URu2Si2. The unique magnetic property is plausibly associated with the mechanism of the unconventional superconductivity.