Variation of Pressure-Induced Valence Transition with Approximation Degree in Yb-Based Quasicrystalline Approximants

We have synthesized new Tsai-type Yb-based intermediate-valence approximant crystals (ACs) with different degree of approximation to quasicrystal, Zn–Au–Yb 1/1 and 2/1 AC, and studied the external pressure effect on their Yb mean-valence $\nu$. Whereas 1/1 AC distinctly exhibits a first-order-like jump in $\nu$ at a transition pressure $P_{\mathrm{v}}$, 2/1 AC only shows an indistinct anomaly at $P_{\mathrm{v}}$. We have also studied the pressure dependence of the $\nu$ of Au–Al–Yb 1/1 AC, which is a prototypal AC exhibiting pressure-induced quantum criticality. It shows a continuous valence anomaly at a critical pressure $P_{\mathrm{c}}$ where the magnetic susceptibility diverges toward zero temperature, in contrast to the valence jump in the Zn–Au–Yb 1/1 AC. These results are discussed based on a theoretical model of quantum critical valence fluctuation.