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To elucidate the ground state of the pressure-stabilized high-temperature ͑HT͒ phase of YbInCu 4 , we have carried out electrical resistivity and ac-susceptibility ac measurements at high pressures. For pressures above 2.49 GPa, the first-order valence transition is completely suppressed ͑below ϳ80 mK). Separately, above 2.39 GPa, a clear peak appears in ac with a small kink in at around T M ϭ2.4 K. The ac peak is easily diminished by applying low magnetic fields and disappears above ϳ500 Oe. The characteristic behavior of ac at T M can generally be ascribed to the onset of long-range ferromagnetic ordering and, therefore, the ground state of the pressure-stabilized HT phase is most probably a ferromagnetically ordered state. This result is compatible with the occurrence of weak ferromagnetism recently reported for the Y-substituted compound Yb 0.8 Y 0.2 InCu 4 under pressure of 1.2 GPa.

(121/123)Sb nuclear quadrupole resonance and muon spin relaxation experiments of Mo_3Sb_7 revealed symmetry breakdown to a nonmagnetic state below the transition recently found at T_S approximately 50 K. The transition is characterized by a distinct lattice dynamics suggested from narrowing of nuclear fields. We point out that the Mo sublattice is a unique three-dimensional frustrated lattice where nearest-neighbor and next-nearest-neighbor antiferromagnetic interactions compete, and propose that tetragonal distortion to release the frustration stabilizes long-range order of spin-singlet dimers, i.e., valence bond crystal, which is thermally excited to the dynamic state with cubic symmetry.

We report high-pressure studies of X-ray diffraction and 11 B-nuclear magnetic resonance (NMR) in the intermediatevalence compound SmB 6 . 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 Q , obtained from the 11 B-NMR measurements, is predominantly contributed by on-site charge distribution. Using the relationship between Q 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.SmB 6 with a cubic CaB 6 -type structure has been intensively studied since the 1960s because of its notable features, for example, an intermediate valence state [Sm valence is $2:6 (see Refs. 1 and 2 and references therein)] and a semiconducting property with a narrow gap (50-100 K 3,4) ). Gap formation in the so-called ''Kondo insulator'' is thought to arise from the hybridization of narrow f bands with a broad conduction band. The insulating gap of SmB 6 is known to be suppressed by the application of pressure, and subsequently, SmB 6 becomes metallic above pressures in the range of 4-7 GPa. 5) Recently, a new magnetically ordered phase has been found above $6 GPa in SmB 6 by highpressure 149 Sm nuclear forward-scattering measurement, so that interest in this compound has been renewed. 6) The physical properties of SmB 6 involve many key factors common to strongly correlated electron systems, such as valence, insulating gap, magnetism, and the Kondo effect.The long-range magnetic order in SmB 6 is expected to result from the localization of Sm 4f electrons on the basis of the general phase diagram of heavy-fermion compounds, that is, the magnetic trivalent state of Sm ions (total angular momentum J ¼ 5=2) becomes more stable than the divalent state (J ¼ 0) under pressure, analogous to the case of Ybbased compounds. The high-pressure properties of this compound have thus far been studied mainly in terms of the P-dependence of the insulating gap by transport measurements. 5,7) However, the microscopic understanding of the P-T phase diagram has been incomplete owing to insufficient experimental information on how the electronic configuration (or valence) varies as pressure increases toward the critical pressure P c of a nonmagnetic-magnetic transition.To address this problem, we carried out nuclear magnetic resonance (NMR) measurements at the B-site under pressure. 8,9) NMR measurement is a powerful tool for probing electronic states through not only local magnetic interactions but also local electric interactions. Recent technical development enables one to perform NMR measurements at very high pressures exceeding 5 GPa. In this letter, we present our latest study of nuclear quadrupole interactions up to 6 G...

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