Pressure-induced anomalous valence crossover in cubic YbCu5-based compounds

Abstract

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.

Figure 1

Schematic of the pressure-temperature phase diagrams and pressure dependence of 4f electron numbers for Yb and Ce systems, where T _N, T _K, and AF are Néel temperature, Kondo temperature, and antiferromagnetic ordered sate, respectively^, . In the Yb system, two quantum critical points (QCPs) are possibly observed. An image of the crystal structure of cubic YbCu₅ is also shown.

Figure 2

The experimental results of cubic YbAg_xCu_5−x (x  = 0, 0.5, and 1.0) at 300 K are shown. (a) X-ray diffraction patterns measured with λ = 0.6888 Å for YbCu₅. (b) Pressure dependence of the volume for YbAg_xCu_5−x (x = 0, 0.5, and 1.0). Solid lines are fits with the equation of state. (c) Pressure dependence of PFY-XAS spectra at 12 K for YbCu₅. (d) Enlarged view of (c) for the quadrupole (QP) and Yb²⁺ components. (e) Yb valence estimated from the fit to the PFY-XAS spectra for cubic YbCu₅ at 300 and 12 K. (f) Pressure dependence of the Yb valence for YbAg_xCu_5−x (x = 0, 0.5, and 1.0) at 300 K.

Figure 3

The experimental results of Yb₂Pd₂Sn are shown. (a) Temperature dependence of the PFY-XAS spectra at ambient pressure. Arrows in (a) correspond to the direction to decrease the temperature. (b) Temperature dependence of the Yb valence estimated from the fits to the PFY-XAS spectra (closed circle) and the RXES spectra (open square) at hv = 8938 eV. (c) Pressure dependence of the PFY-XAS spectra at 20 K. (d) Pressure dependence of the Yb valence estimated from the fits to the PFY-XAS spectra (closed circle) and the RXES spectra (open square). In (d) we also show the pressure dependence of the Néel temperature as a yellow-colored area, where the data are taken from the literature. (e) Crystal structure of Yb₂Pd₂Sn.