Fermi and Non-Fermi Liquid Behavior of Local Moment Systems within a Conserving Slave Boson Theory

TL;DR

This paper develops a conserving slave boson approach to distinguish Fermi liquid and non-Fermi liquid behaviors in local moment systems, accurately capturing the low-energy physics of Anderson impurity models.

Contribution

It introduces a conserving T-matrix approximation that correctly reproduces the spectral exponents indicating Fermi or non-Fermi liquid behavior in Anderson models.

Findings

The method accurately reproduces infrared exponents for Fermi and non-Fermi liquids.

It captures dominant spin flip and charge fluctuation processes in different regimes.

The approach unifies the description of Fermi and non-Fermi liquid behaviors.

Abstract

The question of Fermi liquid vs. non-Fermi liquid behavior induced by strong correlations is one of the prominent problems in metallic local moment systems. As standard models for such systems, the SU(N) x SU(M) Anderson impurity models exhibit both Fermi liquid and non-Fermi liquid behavior, depending on their symmetry. Using an auxiliary boson method, we present a generally applicable scheme to select the relevant contributions in the low frequency regime, while preserving the local gauge symmetry of the model. It amounts to a conserving T-matrix approximation (CTMA) including coherent spin flip as well as charge fluctuation processes, which are found to dominate in the Kondo and in the mixed valence regime, respectively. The infrared threshold exponents of the auxiliary particle spectral functions are indicators for the presence of Fermi or non-Fermi liquid behavior in any given model with strong on-site repulsion. We show that, in contrast to earlier auxiliary boson theories, the CTMA recovers the correct exponents in both cases, indicating that it correctly describes both the Fermi and the non-Fermi regimes of the Anderson model.