Bose-Einstein condensation of magnons in TlCuCl3:: Phase diagram and specific heat from a self-consistent Hartree-Fock calculation with a realistic dispersion relation

We extend the self-consistent Hartree-Fock-Popov calculations by Nikuni et al. [Phys. Rev. Lett. 84 (2000) 58681 concerning the Bose-Eistein condensation of magnons in TlCuCl3 to include a realistic dispersion of the excitations. The result for the critical field as a function of temperature behaves as H-c(T) - H-c(0) similar to T-3/2 below 2 K but deviates from this simple power-law at higher temperature and is in very good agreement with the experimental results. The specific heat is computed as a function of temperature for different values of the magnetic field. It shows a lambda-like shape at the transition and is in good qualitative agreement with the results of Oosawa et al.