Europhys. Lett.
Volume 68, Number 2, October 2004
Page(s) 275 - 281
Section Condensed matter: electronic structure, electrical, magnetic, and optical properties
Published online 01 October 2004
Europhys. Lett., 68 (2), pp. 275-281 (2004)
DOI: 10.1209/epl/i2004-10179-4

Consequences of spin-orbit coupling for the Bose-Einstein condensation of magnons

J. Sirker, A. Weiße and O. P. Sushkov

School of Physics, The University of New South Wales - Sydney 2052, Australia

received 10 May 2004; accepted in final form 11 August 2004

In the first part we discuss how the BEC picture for magnons is modified by anisotropies induced by spin-orbit coupling. In particular, we focus on the effects of antisymmetric spin interactions and/or a staggered component of the g (gyromagnetic) tensor. Such terms lead to a gapped quasiparticle spectrum and a non-zero condensate density for all temperatures so that no phase transition occurs. We contrast this to the effect of crystal field anisotropies which are also induced by spin-orbit coupling. In the second part, we study the field-induced magnetic ordering in $\chem{TlCuCl_3}$ on a quantitative level. We show that the usual BEC picture does not allow for a good description of the experimental magnetisation data and argue that antisymmetric spin interactions and/or a staggered g tensor component are still crucial, although both are expected to be tiny in this compound due to crystal symmetries. Including this type of interaction, we obtain excellent agreement with experimental data.

75.10.Jm - Quantized spin models.
03.75.Hh - Static properties of condensates; thermodynamical, statistical and structural properties.
71.70.Ej - Spin-orbit coupling, Zeeman and Stark splitting, Jahn-Teller effect.

© EDP Sciences 2004