Cumulant expansion for systems with large spins

K. Kladko, P. Fulde, Dmitry A. Garanin

Published Jan 1, 1998• 9 citations

Abstract

(received; accepted) PACS. 05.30.-d – Quantum statistical mechanics. PACS. 75.10.Jm – Quantized spin models. PACS. 75.10.Hk – Classical spin models. Abstract. – A method for obtaining a systematic expansion of thermodynamic functions of spin systems with large spin S in powers of 1/S is proposed. This method uses the cumulant technique and a coherent-state representation of the partition function Z. The expansion of Z in terms of cumulants yields an effective classical Hamiltonian with temperature-dependent quantum corrections. For the Heisenberg quantum Hamiltonian, they have a non-Heisenberg form. The effective Hamiltonian can be solved by methods familiar for classical systems. In recent years considerable interest in condensed matter physics has been focusing on the properties of new magnetic materials, such as magnetic molecules with a large effective spin S, or intermetallic compounds containing magnetic layers or chains. For the latter, most of the theoretical studies deal with S = 1/2 systems which, in the one-dimensional case, allow for analytical solutions using special methods such as the Bethe Ansatz or Jordan-Wigner

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