Bose-Einstein condensation of magnons in TlCuCl$_3$

TL;DR

This paper investigates the magnetic ordering in TlCuCl$_3$ through Bose-Einstein condensation of magnons, revealing that simple models fail and that small interactions significantly alter the physical picture, aligning theory with experiments.

Contribution

It demonstrates that small Dzyaloshinsky-Moriya interactions modify the BEC scenario, providing a more accurate description of magnetic ordering in TlCuCl$_3$.

Findings

Simple BEC models do not match experimental data.

Small interactions induce a nonzero condensate at all temperatures.

Modified models agree well with experimental observations.

Abstract

A quantitative study of the field-induced magnetic ordering in TlCuCl$_3$ in terms of a Bose-Einstein condensation (BEC) of magnons is presented. It is shown that the hitherto proposed simple BEC scenario is in quantitative and qualitative disagreement with experiment. It is further shown that even very small Dzyaloshinsky-Moriya interactions or a staggered $g$ tensor component of a certain type can change the BEC picture qualitatively. Such terms lead to a nonzero condensate density for all temperatures and a gapped quasiparticle spectrum. Including this type of interaction allows us to obtain good agreement with experimental data.