Conserving Diagrammatic Approximations for Quantum Impurity Models: NCA and CTMA

TL;DR

This paper reviews self-consistent diagrammatic methods for quantum impurity models, focusing on the limitations of NCA and how CTMA improves results, especially in Fermi liquid regimes.

Contribution

It introduces the Conserving T-Matrix Approximation (CTMA) as an improved method over NCA for quantum impurity models, addressing NCA's shortcomings.

Findings

CTMA provides qualitatively correct spectral functions.

CTMA accurately captures Fermi liquid behavior.

NCA has fundamental limitations in strong coupling regimes.

Abstract

Self-consistent diagrammatic approximations to the Anderson or Kondo impurity model, using an exact pseudoparticle representation of the impurity states, are reviewed. We first discuss the infrared exponents of the pseudoparticle propagators as indicators of Fermi liquid behavior through their dependence on the impurity occupation and on magnetic field. Then we discuss the Non-Crossing Approximation (NCA), identifying its strengths, but also its fundamental shortcomings. Physical arguments as well as a perturbative renormalization group analysis suggest that an infinite parquet-type resummation of two-particle vertex diagrams, the Conserving T-Matrix Approximation (CTMA) will cure the deficiencies of NCA. We review results on the pseudoparticle spectral functions, the spin susceptibility and the impurity electron spectral function, supporting that the CTMA provides qualitatively correct results, both in the high-temperature regime and in the strong coupling Fermi liquid regime at low temperatures.