Theory of many-fermion systems

TL;DR

This paper develops a comprehensive field-theoretical framework for many-fermion systems, incorporating disorder and interactions, and analyzes phase transitions, fixed points, and corrections using advanced techniques like bosonization, symmetry analysis, and renormalization group methods.

Contribution

It introduces a unified matrix field theory approach that generalizes existing approximations and derives a nonlinear sigma-model for disordered Fermi liquids, extending understanding of phase transitions and fixed points.

Findings

Disordered Fermi-liquid fixed point exists and is stable for d>2.

Weak-localization effects are understood as corrections near the fixed point.

Systematic expansion improves upon RPA and confirms Fermi-liquid stability for d>1.

Abstract

A general field-theoretical description of many-fermion systems, with or without quenched disorder, is developed. Starting from the Grassmannian action for interacting fermions, we first bosonize the theory by introducing composite matrix variables that correspond to two-fermion excitations and integrating out the fermion degrees of freedom. The saddle point solution of the resulting matrix field theory reproduces a disordered Hartree-Fock approximation and an expansion to Gaussian order about the saddle point corresponds to a disordered RPA-like theory. In the clean limit they reduce to the ordinary Hartree-Fock and random-phase approximations. We first concentrate on disordered systems, and perform a symmetry analysis that allows for a systematic separation of the massless modes from the massive ones. By treating the massive modes in a simple approximation, one obtains a technically satisfactory derivation of the generalized nonlinear sigma-model that has been used in the theory of metal-insulator transitions. The theory also allows for the treatment of other phase transitions in the disordered Fermi liquid. We further use renormalization group techniques to establish the existence of a disordered Fermi-liquid fixed point, and show that it is stable for all dimensions d>2. The so-called weak-localization effects can be understood as corrections to scaling near this fixed point. The general theory also allows for studying the clean limit. For that case we develop a loop expansion that corresponds to an expansion in powers of the screened Coulomb interaction, and that represents a systematic improvement over RPA. We corroborate the existence of a Fermi-liquid fixed point that is stable for all dimensions d>1, in agreement with other recent field-theoretical treatments of clean Fermi liquids.