Emergence of Spin Order in Two-Dimensional Quantum Heisenberg Antiferromagnets

TL;DR

This study uses effective field theory to analyze how two-dimensional quantum Heisenberg antiferromagnets exhibit unusual temperature-dependent magnetic behaviors under different magnetic field configurations, revealing novel order-disorder phenomena.

Contribution

It provides a systematic theoretical analysis of spin order emergence in 2D quantum antiferromagnets under aligned and orthogonal magnetic fields, uncovering non-monotonic magnetization behaviors.

Findings

Finite-temperature uniform magnetization grows with temperature in aligned fields.

In orthogonal fields, magnetization first decreases, then increases with temperature.

Staggered magnetization and entropy density show non-monotonic temperature dependence.

Abstract

Counterintuitive order-disorder phenomena emerging in antiferromagnetically coupled spin systems have been reported in various studies. Here we perform a systematic effective field theory analysis of two-dimensional bipartite quantum Heisenberg antiferromagnets subjected to either mutually aligned -- or mutually orthogonal -- magnetic and staggered fields. Remarkably, in the aligned configuration, the finite-temperature uniform magnetization $M_T$ grows as temperature rises. Even more intriguing, in the orthogonal configuration, $M_T$ first drops, goes through a minimum, and then increases as temperature rises. Unmasking the effect of the magnetic field, we furthermore demonstrate that the finite-temperature staggered magnetization $M^H_s$ and entropy density -- both exhibiting non-monotonic temperature dependence -- are correlated. Interestingly, in the orthogonal case, $M^H_s$ presents a maximum, whereas in mutually aligned magnetic and staggered fields, $M^H_s$ goes through a minimum. The different behavior can be traced back to the existence of an "easy XY-plane" that is induced by the magnetic field in the orthogonal configuration.