Renormalization-group study of Anderson and Kondo impurities in gapless Fermi systems

TL;DR

This paper uses renormalization-group analysis to study thermodynamic properties of magnetic impurity models in gapless Fermi systems, revealing how energy-dependent scattering rates influence Kondo screening and fixed points.

Contribution

It provides a detailed RG analysis of impurity models with energy-dependent scattering rates, highlighting the effects on local-moment behavior and fixed points in gapless systems.

Findings

Suppression of mixed valence due to vanishing scattering rate at Fermi level.

Identification of intermediate-coupling fixed points in various Kondo models.

Role of particle-hole symmetry in impurity behavior and fixed point stability.

Abstract

Thermodynamic properties are presented for four magnetic impurity models describing delocalized fermions scattering from a localized orbital at an energy-dependent rate $\Gamma(\epsilon)$ which vanishes precisely at the Fermi level, $\epsilon = 0$. Specifically, it is assumed that for small $|\epsilon|$, $\Gamma(\epsilon)\propto|\epsilon|^r$ with $r>0$. The cases $r=1$ and $r=2$ describe dilute magnetic impurities in unconventional superconductors, ``flux phases'' of the two-dimensional electron gas, and zero-gap semiconductors. For the nondegenerate Anderson model, the depression of the low-energy scattering rate suppresses mixed valence in favor of local-moment behavior, and leads to a marked reduction in the exchange coupling on entry to the local-moment regime, with a consequent narrowing of the range of parameters within which the impurity spin becomes Kondo-screened. The relationship between the Anderson model and the exactly screened Kondo model with power-law exchange is examined. The intermediate-coupling fixed point identified in the latter model by Withoff and Fradkin (WF) has clear signatures in the thermodynamic properties and in the local magnetic response of the impurity. The underscreened, impurity-spin-one Kondo model and the overscreened, two-channel Kondo model both exhibit a conditionally stable intermediate-coupling fixed point in addition to unstable fixed points of the WF type. In all four models, the presence or absence of particle-hole symmetry plays a crucial role.