FLEX-description of the spectral functions near singlet-triplet transition

TL;DR

This paper applies the FLEX approximation to analyze spectral functions near the singlet-triplet transition in a non-equilibrium two-level Anderson model, emphasizing its ability to accurately capture the Kondo temperature and spectral features.

Contribution

It introduces the use of the FLEX approximation for this model, demonstrating its effectiveness over previous iterative perturbation methods in describing spectral properties and the Kondo scale.

Findings

FLEX accurately describes the Kondo temperature $T_K$.

Spectral features near the singlet-triplet transition are well captured.

FLEX provides a conserving approximation suitable for non-equilibrium conditions.

Abstract

In a previous article, we have investigated the non-equilibrium two-level Anderson model with a simple iterative perturbation theory. Here we use here a more sophisticated perturbative method, the fluctuation-exchange (FLEX) approximation. The great advantage of FLEX is its \textit{conserving} nature, and that it can describe well the Kondo energy scale, the Kondo-temperature, $T_{K}$. As it was expected from the results obtained with iterative perturbation theory, the FLEX description can give back also the relevant features of the spectral properties.