Quantum Critical Behavior in Kondo Systems

TL;DR

This paper reviews quantum critical behavior in heavy-fermion Kondo systems, focusing on an extended dynamical mean-field theory, types of quantum phase transitions, and experimental tests involving neutron scattering.

Contribution

It introduces an extended dynamical mean-field theory for the Kondo lattice and identifies conditions for local quantum critical points, supported by experimental evidence.

Findings

Identification of two types of quantum phase transitions in Kondo systems.

Existence of a local quantum critical point with anomalous spin susceptibility.

Neutron scattering evidence supporting local quantum critical behavior in CeCu₆-xAux.

Abstract

This article briefly reviews three topics related to the quantum critical behavior of certain heavy-fermion systems. First, we summarize an extended dynamical mean-field theory for the Kondo lattice, which treats on an equal footing the quantum fluctuations associated with the Kondo and RKKY couplings. The dynamical mean-field equations describe an effective Kondo impurity model with an additional coupling to vector bosons. Two types of quantum phase transition appear to be possible within this approach---the first a conventional spin-density-wave transition, the second driven by local physics. For the second type of transition to be realized, the effective impurity model must have a quantum critical point exhibiting an anomalous local spin susceptibility. In the second part of the paper, such a critical point is shown to occur in two variants of the Kondo impurity problem. Finally, we propose an operational test for the existence of quantum critical behavior driven by local physics. Neutron scattering results suggest that CeCu$_{6-x}$Au$_x$ passes this test.