Hidden zero-temperature bicritical point in the 2D anisotropic Heisenberg model: Monte Carlo simulations and novel finite-size scaling*

The bicritical point in the phase diagram of the 2D anisotropic Heisenberg antiferromagnet in a field has not been fully resolved by Monte Carlo simulations. A recent study [Phys. Rev. B \textbf{72}, 064443 (2005)] showed an upper bound for the bicritical temperature. By performing quite detailed Monte Carlo simulations near the apparent spin-flop line, we found this system was governed by a single-spin Hamiltonian, which terminates the renormalization group flow of a finite-size 2D nonlinear $\sigma $ model. Using a novel finite-size scaling analysis, we confirm that the bicritical point in two dimensions is Heisenberg-like and occurs at T=0. Thus, the uncertainty in the phase diagram is completely removed [Phys. Rev. B \textbf{74}, 064407 (2006)].