Quantum critical region of a two-dimensional spin-half XXZ model

We study the properties of a quantum critical region of an XXZ model in two dimensions. The quantum critical point at zero temperature evolves as the temperature increases. Using the quantum Monte Carlo method with stochastic series expansions, we simulated the lattices of which size are up to 200 × 200 . For the finite-temperature transitions from a paramagnet to an XY critical phase, we measured the superfluid stiffness and used the finite-size scaling to obtain the critical temperatures. For the paramagnet-to-antiferromagnetic transitions, we measured the compressibility to obtain the critical points. With these critical points, we constructed the finite-temperature phase diagram of the XXZ model in two dimensions.