Quantum Berezinskii-Kosterlitz-Thouless transition in square-lattice magnets with easy-plane anisotropy

We consider the quantum easy-plane Heisenberg magnet on the square lattice. After the introduction of a suitable classical effective Hamiltonian, computer simulations, both for ferro- and antiferromagnets, were performed to obtain the spin correlation lengths. The effects of quantum fluctuations are quantitatively evaluated and discussed for different values of the spin, in a broad range of temperatures, above the Berezinskii-Kosterlitz-Thouless transition.