Auxiliary-Fermion Approach to Critical Fluctuations in the 2D Quantum AF Heisenberg Model

TL;DR

This paper develops an auxiliary-fermion approach to study critical fluctuations in the 2D quantum antiferromagnetic Heisenberg model, introducing a minimal self-consistent approximation that captures dynamical scaling and short-range order effects.

Contribution

It proposes a new minimal self-consistent approximation (MSCA) within the auxiliary-fermion framework to better describe dynamical properties and short-range order in the 2D quantum AF Heisenberg model.

Findings

MSCA captures dynamical scaling at low energies and temperatures.

Static properties like correlation length are accurately reproduced.

Overdamping issues in previous approximations are addressed.

Abstract

The nearest-neighbor quantum-antiferromagnetic (AF) Heisenberg model for spin 1/2 on a two-dimensional square lattice is studied in the auxiliary-fermion representation. Expressing spin operators by canonical fermionic particles requires a constraint on the fermion charge Q=1 on each lattice site, which is imposed approximately through the thermal average. The resulting interacting fermion system is first treated in mean-field theory (MFT), which yields an AF ordered ground state and spin waves in quantitative agreement with conventional spin-wave theory. At finite temperature a self-consistent approximation beyond mean field is required in order to fulfill the Mermin-Wagner theorem. We first discuss a fully self-consistent approximation, where fermions are renormalized due to fluctuations of their spin density, in close analogy to FLEX. While static properties like the correlation length come out correctly, the dynamical response lacks the magnon-like peaks which would reflect the appearance of short-range order at low T. This drawback, which is caused by overdamping, is overcome in a `minimal self-consistent approximation' (MSCA), which we derive from the equations of motion. The MSCA features dynamical scaling at small energy and temperature and is qualitatively correct both in the regime of order-parameter relaxation at long wavelengths and in the short-range-order regime. We also discuss the impact of vertex corrections and the problem of pseudo-gap formation in the single-particle density of states due to long-range fluctuations. Finally we show that the (short-range) magnetic order in MFT and MSCA helps to fulfill the constraint on the local fermion occupancy.