The Extended Coupled Cluster Treatment of Correlations in Quantum Magnets

TL;DR

This paper applies the extended coupled cluster method to quantum spin models, successfully describing different phases and providing improved numerical results for magnetization in quantum magnets.

Contribution

It introduces an extended coupled cluster approach that systematically incorporates correlations in quantum magnets, improving upon previous methods.

Findings

Accurately describes Ising-Heisenberg and XY-Heisenberg phases

Provides the best non-extrapolated sublattice magnetization results

Reveals different behaviors in chain and lattice geometries

Abstract

The spin-half XXZ model on the linear chain and the square lattice are examined with the extended coupled cluster method (ECCM) of quantum many-body theory. We are able to describe both the Ising-Heisenberg phase and the XY-Heisenberg phase, starting from known wave functions in the Ising limit and at the phase transition point between the XY-Heisenberg and ferromagnetic phases, respectively, and by systematically incorporating correlations on top of them. The ECCM yields good numerical results via a diagrammatic approach, which makes the numerical implementation of higher-order truncation schemes feasible. In particular, the best non-extrapolated coupled cluster result for the sublattice magnetization is obtained, which indicates the employment of an improved wave function. Furthermore, the ECCM finds the expected qualitatively different behaviours of the linear chain and the square lattice cases.