Pressure-Driven Quantum Criticality and T/H Scaling in the Icosahedral Au-Al-Yb Approximant

We report on ac magnetic susceptibility measurements under pressure of the Au-Al-Yb alloy, a crystalline approximant to the icosahedral quasicrystal that shows unconventional quantum criticality. In describing the susceptibility as $chi(T)^{-1} - chi(0)^{-1} propto T^{gamma}$, we find that $chi(0)^{-1}$ decreases with increasing pressure and vanishes to zero at the critical pressure $P_{rm c} simeq 2$ GPa, with $gamma~ (simeq 0.5)$ unchanged. We suggest that this quantum criticality emerges owing to critical valence fluctuations. Above $P_{rm c}$, the approximant undergoes a magnetic transition at $T simeq 100$ mK. These results are contrasted with the fact that, in the quasicrystal, the quantum criticality is robust against the application of pressure. The applicability of the so-called $T/H$ scaling to the approximant is also discussed.