Local-moment formation in gapless Fermi systems

TL;DR

This paper uses perturbative scaling to analyze the Anderson model with a density of states that varies as |E|^r, revealing how pseudo-gaps influence local-moment formation and Kondo screening in unconventional superconductors and semiconductors.

Contribution

It extends the Anderson model analysis to systems with gapless spectra, showing how pseudo-gaps affect impurity magnetic behavior and Kondo effects.

Findings

Pseudo-gap suppresses mixed valence states.

Reduces exchange coupling J in local-moment regime.

Decreases the parameter range for Kondo screening.

Abstract

Perturbative scaling is applied to the Anderson model for a localized level coupled to a Fermi system in which the density of states varies like $|E|^r$ near the Fermi energy ($E=0$). This model with $r=1$ or $2$ may describe magnetic impurities in unconventional superconductors and certain semiconductors. The pseudo-gap is found to suppress mixed valence in favor of local-moment behavior. However, the magnitude of the exchange coupling $J$ in the local-moment regime is reduced, thereby decreasing the parameter range within which the impurity becomes Kondo-screened at low temperatures.