Quantum Critical Metals: beyond the Order Parameter Fluctuations

TL;DR

This paper discusses the limitations of traditional quantum critical point theories in metals, emphasizing the role of non-Fermi liquid behavior and local quantum criticality, especially in heavy fermion systems.

Contribution

It highlights the breakdown of order parameter fluctuation models and introduces the concept of locally quantum critical behavior in heavy fermion metals.

Findings

Non-Fermi liquid excitations emerge at quantum critical points.

Destruction of the Kondo effect characterizes certain quantum critical metals.

Theoretical and experimental evidence supports locally quantum critical behavior.

Abstract

The standard description of quantum critical points takes into account only fluctuations of the order parameter, and treats quantum fluctuations as extra dimensions of classical fluctuations. This picture can break down in a qualitative fashion in quantum critical metals: non-Fermi liquid electronic excitations are formed precisely at the quantum critical point and appear as a part of the quantum-critical spectrum. In the case of heavy fermion metals, it has been proposed that the non-Fermi liquid behavior is characterized by the destruction of the Kondo effect. The latter invalidates Hertz's Gaussian theory of paramagnons and leads to an interacting theory that is ``locally quantum critical''. We summarize the theoretical and experimental developments on the subject. We also discuss their broader implications, and make contact with recent work on quantum critical magnets.