Critical Temperature and the Transition from Quantum to Classical Order Parameter Fluctuations in the Three-Dimensional Heisenberg Antiferromagnet

We present results of extensive quantum Monte Carlo simulations of the three-dimensional $S=1\mathrm{}/2$ Heisenberg antiferromagnet. Finite-size scaling of the spin stiffness and the sublattice magnetization gives the critical temperature $T_{\mathrm{c}}/J=0.946\mathrm{}\pm 0.001$. The critical behavior is consistent with the classical 3D Heisenberg universality class, as expected. We discuss the general nature of the transition from quantum mechanical to classical (thermal) order parameter fluctuations at a continuous $T_{\mathrm{c}}>0\mathrm{}$ phase transition.