Quasiclassical Hamiltonians for large-spin systems
D. A. Garanin, K. Kladko, P. Fulde
Abstract
We propose a method for obtaining effective classical Hamiltonians H for many-body quantum spin systems with large spins. This method uses the coherent-state representation of the partition function Z and the cumulant expansion in powers of 1/S. For the quantum Hamiltonian H of a Heisenberg form, the 1/S corrections in H have a non-Heisenberg many-spin form. The effective Hamiltonian H can be treated by methods familiar for classical systems. The non-Heisenberg terms in H may be responsible for such effects as spin-Peierls transition and uplifting of the classical degeneracy by quantum fluctuations.
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