2-DIMENSIONAL SPIN-1/2 HEISENBERG-ANTIFERROMAGNET AT FINITE TEMPERATURE

We analyzed the high-temperature series of the two-dimensional spin-1/2 Heisenberg antiferromagnet using a variant of the Pade approximant method. Our analysis agrees very well with the Monte Carlo simulation result. Specifically, the agreement is very good for the internal energy per spin at T > 0.3J, for the specific heat and for the uniform susceptibility at T > 0.5J on a square lattice. We also analyzed the internal-energy series on a triangular lattice. The ground-state energy per spin at zero temperature is found to be E(g)/N = -0.524, which is in good agreement with the values obtained by simulations and spin-wave theory.