Unconventional Quantum Criticality due to Critical Valence Transition

TL;DR

This paper reviews how valence transition-induced quantum criticality in heavy fermion metals explains non-Fermi liquid behavior and anomalies in physical properties, highlighting the role of critical valence fluctuations beyond magnetic quantum criticality.

Contribution

It provides a comprehensive review of experimental and theoretical developments on unconventional quantum criticality driven by valence transitions in heavy fermion systems.

Findings

Critical valence fluctuations explain non-Fermi liquid behavior.

Anomalies in resistivity and superconductivity linked to valence changes.

Unified understanding of physical quantity anomalies around valence transition points.

Abstract

Quantum criticality due to the valence transition in some Yb-based heavy fermion metals has gradually turned out to play a crucial role to understand the non-Fermi liquid properties that cannot be understood from the conventional quantum criticality theory due to magnetic transitions. Namely, critical exponents giving the temperature (T) dependence of the resistivity \rho(T), the Sommerfeld coefficient, C(T)/T, the magnetic susceptibility, \chi(T), and the NMR relaxation rates, 1/(T_{1}T), can be understood as the effect of the critical valence fluctuations of f electrons in Yb ion in a unified way. There also exist a series of Ce-based heavy fermion metals that exhibit anomalies in physical quantities, enhancements of the residual resistivity \rho_{0} and the superconducting critical temperature (T_c) around the pressure where the valence of Ce sharply changes. Here we review the present status of these problems both from experimental and theoretical aspects.