In this article, we review high-pressure nuclear magnetic resonance studies on SmS, a prototypical intermediate valence compound, which was achieved using a ³³S-enriched sample. A resonance arising from the magnetically ordered phase has been for the first time observed below 15-20 K above a nonmagnetic-magnetic transition pressure Pc₂ ≈ 2 GPa. We have obtained information on the first-order nature of the transition. Also, the screening of the localized character of Sm-4f electrons is suggested, which may require an understanding beyond the conventional framework for heavy fermions. From the fact that spontaneous internal fields cancel out at the S site leads, we conclude that the ordered phase has a type II antiferromagnetic structure.

In this article, we review the recent progress in our understanding of electronic states in SmS, Yb-based quasicrystals and approximants, which exhibit valence-fluctuations. Results of partial-fluorescence-yield X-ray absorption spectroscopy experiments are presented. We also present the intriguing relationship between the rare-earth meanvalence and the thermodynamic properties, with an emphasis on the discussion of the possible origin of the unconventional quantum critical behavior in the Yb-based quasicrystals.

To clarify a high-pressure cubic valence fluctuating ground state of YbPd appearing after collapse of valence order, powder X-ray diffraction and electrical resistivity under high pressures were measured at low temperatures. Tracing splitting of a (4 0 0) Bragg peak manifests that the structural phase transition into a tetragonal symmetry is shifted linearly to 20 K at P = 4.4 GPa and is suppressed at P = 5.0 GPa. The resistivity measurements reveal non-Fermi liquid

(NFL) behavior and no emergence of new magnetic order. The resistivity exhibits T-linear dependence above 8 GPa, which corresponds to quantum valence fluctuation.

We have investigated the effect of pressure on the electronic state of antiferromagnetic compound EuCu₂Ge₂ by electrical resistivity measurement up to 10 GPa using a cubic anvil cell apparatus. On applying pressure, the Néel temperature of 𝑇_N=15 K at ambient pressure increases monotonically and reaches a maximum value 27 K at 6.2 GPa.

Thereafter, however, 𝑇_N suddenly drops to zero at 6.5 GPa, suggesting the quantum critical point driven by the valence transition of Eu from a nearly divalent (Eu²⁺) to a nonmagnetic trivalent (Eu³⁺) state. Here we would like to mention that this result is the first observation of a valence transition among the Eu-based compounds.

This article reviews our studies of unusual magnetic orderings in Yb intermetallic compounds YbNi₃Ga₉ and YbAgGe using high-pressure specific-heat measurements. Upon applying pressure above 9 GPa, YbNi₃Ga₉ with a chiral trigonal structure undergoes a magnetic order at 1.5 K. Application of magnetic field B⊥c induces another ordered phase in the B-T phase diagrams at P > 11 GPa. The origin of this induced phase is discussed in relation to the chiral soliton lattice. YbAgGe with a quasi-kagome lattice of Yb ions undergoes two magnetic transitions at 𝑇_M1=0.8 K and 𝑇_M2=0.65 K at ambient pressure. For 0.5 < P < P* = 1.7 GPa, the TM remains constant at 0.85 K, while the T_M increases linearly with P > P*. The magnetic entropy at T_M rises for P > P*, whereas the Kondo temperature T_K does not change. These findings suggest that the sudden rise of T_M(P) for P > P* is not due to the lowering of T_K but the release of the magnetic frustration in the quasi-kagome lattice.

A review is given on our recent optical conductivity [σ(ω)] studies for heavy fermion Ce and Yb compounds under high pressure. Diamond anvil cells were used to generate high pressures and synchrotron radiation at SPring-8 was used as a bright infrared source. In the measured σ(ω) spectra, a marked peak is observed in the mid-infrared range due to a high density of 4f-derived states near the Fermi level. The infrared peak exhibits significant shifts toward high and low energies in Ce and Yb systems, respectively. These opposite tendencies in Ce and Yb systems are discussed in terms of pressure-induced shifts of the 4f level, and in terms of the electron-hole symmetry.