Bose-Einstein condensation in antiferromagnets close to the saturation field

TL;DR

This paper demonstrates that the quantum phase transition in a specific antiferromagnet under high magnetic fields can be understood as a Bose-Einstein condensation of magnons, supported by theoretical modeling and experimental data analysis.

Contribution

The study models the transition at the critical field as magnon BEC using a bond operator approach and confirms this with experimental magnetization data in a real material.

Findings

Magnetization scaling matches theoretical BEC predictions

Quantum critical exponents are obtained from data

Transition at Hc2 confirmed as magnon BEC

Abstract

At zero temperature and strong applied magnetic fields the ground sate of an anisotropic antiferromagnet is a saturated paramagnet with fully aligned spins. We study the quantum phase transition as the field is reduced below an upper critical $H_{c2}$ and the system enters a XY-antiferromagnetic phase. Using a bond operator representation we consider a model spin-1 Heisenberg antiferromagnetic with single-ion anisotropy in hyper-cubic lattices under strong magnetic fields. We show that the transition at $H_{c2}$ can be interpreted as a Bose-Einstein condensation (BEC) of magnons. The theoretical results are used to analyze our magnetization versus field data in the organic compound $NiCl_2$-$4SC(NH_2)_2$ (DTN) at very low temperatures. This is the ideal BEC system to study this transition since $H_{c2}$ is sufficiently low to be reached with static magnetic fields (as opposed to pulsed fields). The scaling of the magnetization as a function of field and temperature close to $H_{c2}$ shows excellent agreement with the theoretical predictions. It allows to obtain the quantum critical exponents and confirm the BEC nature of the transition at $H_{c2}$.