Geometry fluctuations in a two-dimensional quantum antiferromagnet

TL;DR

This study investigates how geometric fluctuations, specifically phason flip disorder, influence the ground state and antiferromagnetic order in a two-dimensional quantum Heisenberg antiferromagnet, revealing an order-by-disorder phenomenon.

Contribution

It introduces the concept of phason flip disorder in quantum antiferromagnets and demonstrates its counterintuitive effect of enhancing antiferromagnetic order.

Findings

Antiferromagnetism is enhanced by phason disorder.

The staggered order parameter increases with defects.

Ground state energy also increases with disorder.

Abstract

The paper considers the effects of random fluctuations of the local spin connectivities (fluctuations of the geometry) on ground state properties of a two-dimensional quantum antiferromagnet. We analyse the behavior of spins described by the Heisenberg model as a function of what we call phason flip disorder, following a terminology used for aperiodic systems. The calculations were carried out both within linear spin wave theory and using quantum Monte Carlo simulations. An "order by disorder" phenomenon is observed in this model, wherein antiferromagnetism is found to be enhanced by phason disorder. The value of the staggered order parameter increases with the number of defects, accompanied by an increase in the ground state energy of the system.