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Regular version of the site

Article

Equilibrium and nonequilibrium mean-field dynamics of quantum spin cluster

Journal of Experimental and Theoretical Physics. 1994. Vol. 79. No. 5. P. 824-832.
Kolokolov I., Chertkov M.
Using the functional integration method we compute exactly the time-dependent correlation functions of local spins in a finite quantum Heisenberg magnet with a position-independent exchange between spins. Explicit expressions for such correlation functions for an arbitrary number (N) of spins in the cluster and for all temperatures are presented. The spin correlation function for a cluster placed in an external time-dependent magnetic field is obtained under the same conditions. At large N the correlation function is found to have a Gaussian bump and a nonvanishing tail at large times. It is in good agreement with the finite-time domain of the experimental curve. A possible application of the exact results, which form a starting dynamical mean-field approximation for long-range magnets and an accompanied physical picture, are discussed