Sound attenuation on the Bose-Einstein condensation of magnons in TlCuCl_3

TL;DR

This study explores sound attenuation in TlCuCl_3 near magnon Bose-Einstein condensation, revealing a sharp attenuation peak, hysteresis, and magnon-lattice coupling effects, combining experimental and theoretical insights into quantum spin dynamics.

Contribution

It provides the first combined experimental and theoretical analysis of sound attenuation and magnon-lattice interactions near BEC in TlCuCl_3, highlighting a first-order transition feature.

Findings

Sharp sound attenuation peak near BEC transition

Hysteresis indicates first-order transition behavior

Magnon-lattice coupling estimated from data

Abstract

We investigated experimentally and theoretically sound attenuation in the quantum spin system TlCuCl_3 in magnetic fields at low temperatures. Near the point of Bose-Einstein condensation (BEC) of magnons a sharp peak in the sound attenuation is observed. The peak demonstrates a hysteresis as function of the magnetic field pointing to a first-order contribution to the transition. The sound damping has a Drude-like form arising as a result of hardcore magnon-magnon collisions. The strength of the coupling between lattice and magnons is estimated from the experimental data. The puzzling relationship between the transition temperature and the concentration of magnons is explained by their "relativistic" dispersion.