Cumulant expansion for systems with large spins

¹ Condensed Matter Physics Group and Center for Nonlinear Studies Theoretical Division, Los Alamos National Laboratory MS-B258, Los Alamos, New Mexico 87545, USA
² Max-Planck-Institut für Physik komplexer Systeme Nöthnitzer Str. 38, D-01187 Dresden, Germany

Received: 23 November 1998
Accepted: 12 March 1999

Abstract

A method is proposed for obtaining a systematic expansion of thermodynamic functions of spin systems with large spin S in powers of 1/S. It uses the cumulant technique and a coherent-state representation of the partition function . The expansion of 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.