Macroscopic properties of triplon Bose-Einstein condensates

Abstract

Magnetic insulators can be characterized by a gap separating the singlet ground state from the lowest energy triplet, S=1 excitation. If the gap can be closed by the Zeeman interaction in applied magnetic field, the resulting S=1 quasiparticles, triplons, can have concentrations sufficient to undergo the Bose-Einstein condensates transition. We consider macroscopic properties of the triplon Bose-Einstein condensates in the Hartree-Fock-Bogoliubov approximation taking into account the anomalous averages. We prove that these averages play the qualitative role in the condensate properties. As a result, we show that with the increase in the external magnetic field at a given temperature, the condensate demonstrates an instability related to the appearance of nonzero phonon damping and a change in the characteristic dependence of the speed of sound on the magnetic field. The calculated magnetic susceptibility diverges when the external magnetic field approaches this instability threshold, providing a tool for the experimental verification of this approach.

published version

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• The effects of disorder in dimerized quantum magnets in mean field approximations
• Spin-gapped magnets with weak anisotropies I: Constraints on the phase of the condensate wave function
• Effects of exchange and weak Dzyaloshinsky-Moriya anisotropies on thermodynamic characteristics of spin-gapped magnets
• Critical behavior of Tan's contact for bosonic systems with a fixed chemical potential
• Phase Transitions in Three-Dimensional Bosonic Systems in Optical Lattices
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• Defining a critical temperature of a crossover from BEC to the normal phase in anisotropic quantum magnets