Bose-Einstein condensation without phase transition

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  • 12 March 2004
Abstract
The Bose-Einstein condensation (BEC) of magnons in spin-gap systems is considered. It is shown that antisymmetric spin interactions induced by spin-orbit coupling are essential to obtain the generic scenario. Such terms lead to a gapped quasiparticle spectrum and a nonzero condensate density for all temperatures so that no phase transition occurs. As example the field-induced magnetic ordering in TlCuCl$_3$ is investigated and it is shown that the hitherto proposed simple BEC scenario disagrees with experiment. Antisymmetric spin interactions, although tiny in this compound due to crystal symmetries, turn out to be still crucial for a correct theoretical description.

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