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We report electrical resistivity, ac magnetic susceptibility, and x-ray absorption spectroscopy measurements of intermediate valence YbNi_{3}Ga_{9} under pressure and magnetic field. We have revealed a characteristic pressure-induced Yb valence crossover within the temperature-pressure phase diagram, and a first-order metamagnetic transition is found below P_{c}∼9  GPa where the system undergoes a pressure-induced antiferromagnetic transition. As a possible origin of the metamagnetic behavior, a critical valence fluctuation emerging near the critical point of the first-order valence transition is discussed on the basis of the temperature-field-pressure phase diagram.

We have performed hard x-ray photoemission spectroscopy (HAXPES) for Yb-based Kondo lattice compounds; an antiferromagnetic heavy-fermion system YbNi 3 Al 9 and a valence fluctuation system YbNi 3 Ga 9. The Yb 3d 5/2 spectra of YbNi 3 Ga 9 showed both Yb 2+ and Yb 3+-derived structures indicating strong valence fluctuation, and the intensity of Yb 2+ (Yb 3+) structures gradually increased (decreased) on cooling. The Yb 3d 5/2 spectra of YbNi 3 Al 9 mostly consisted of Yb 3+-derived structures and showed little temperature dependence. The Yb valences of YbNi 3 Ga 9 and YbNi 3 Al 9 at 22 K were evaluated to be 2.43 and 2.97, respectively. Based on the results of the Ni 2p and valence-band HAXPES spectra together with soft x-ray valence-band spectra, we described that the difference of physical properties of YbNi 3 X 9 (X = Al, Ga) is derived from the differences of the 4f-hole level relative to the Fermi level (E F) and Ni 3d density of states at E F. The HAXPES results on the Yb valences were consistent with those obtained by x-ray absorption spectroscopy using the partial fluorescence yield mode and resonant x-ray emission spectroscopy at the Yb L 3 edge.

We report specific-heat and magnetocaloric-effect studies on single-crystalline Kondo helical magnet YbNi 3 Al 9 . Molecular field analysis of a Schottky peak due to the Zeeman splitting of the Yb-ion doublet crystalline-field ground state demonstrates that the interlayer antiferromagnetic exchange interactions are 2 orders of magnitude smaller than that of the intralayer ferromagnetic coupling among Yb ions, reflecting realization of magnetically well separated Yb layers. The Sommerfeld coefficient γ , which is 110 mJ/K 2 mol in zero field, decreases smoothly with increasing field without any noticeable anomalies at the helical magnetic phase boundary. This fact confirms that heavy quasiparticles are formed on a part of the Fermi surface away from "hot sheets" that have nesting instabilities responsible for the helical magnetic structure. These results indicate that YbNi 3 Al 9 is a novel system where heavy quasiparticles are confined within the two-dimensional Yb layers.