Thermodynamics of the quantum easy-plane antiferromagnet on the triangular lattice

The classical XXZ triangular-lattice antiferromagnet shows both an Ising and a Berezinskii-Kosterlitz-Thouless transition, related to the chirality and the in-plane spin components, respectively. In this paper the quantum effects on the thermodynamic quantities are evaluated by means of the pure-quantum self-consistent harmonic approximation, which allows one to deal with any spin value through classical Monte Carlo simulations. We present data for the internal energy, the specific heat, the static spin correlation functions, and the in-plane correlation length for different values of the spin and of the exchange anisotropy. The quantum transition temperatures turn out to be smaller the smaller the spin, and agree with the few available theoretical and numeric al estimates. [S0163-1829(99)04234-4].