Pressure-induced anomalous valence crossover in cubic YbCu-based compounds.

A pressure-induced anomalous valence crossover without structural phase transition is observed in archetypal cubic YbCu based heavy Fermion systems. The Yb valence is found to decrease with increasing pressure, indicating a pressure-induced crossover from a localized 4f 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 YbCu-based compounds. The intermetallics YbPdSn, which shows two quantum critical points (QCP) under pressure and has been proposed as a potential candidate for a reentrant Yb 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 Yb state at the second QCP.