Thermodynamics of the quantum easy-plane antiferromagnet on the triangularlattice

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