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Monte Carlo computation of the free energy in quantum two-dimensional Heisenberg ferromagnets using the expanded-ensemble method

The expanded-ensemble method, initially formulated for classical systems, is combined with the Handscomb quantum Monte Carlo approach to develop a new method for the free-energy MC calculation of quantum systems. It was applied to a quantum 2D Heisenberg ferromagnetic system and proved effective. Comparisons were performed between the Takahashi spin-wave approximation for free energy and the authors' numerical results.