Quantum valence criticality as an origin of unconventional critical phenomena.

It is shown that unconventional critical phenomena commonly observed in paramagnetic metals YbRh2Si2, YbRh2(Si0.95Ge0.05)2, and β-YbAlB4 are naturally explained by the quantum criticality of Yb-valence fluctuations. We construct the mode-coupling theory taking account of local correlation effects of f electrons and find that unconventional criticality is caused by the locality of the valence fluctuation mode. We show that measured low-temperature anomalies such as divergence of uniform spin susceptibility χ∼T(-ζ) with ζ∼0.6 giving rise to a huge enhancement of the Wilson ratio and the emergence of T-linear resistivity are explained in a unified way.