Two-Dimensional Heisenberg Model with Spin s=1/2 and Antiferromagnetic Exchange Treated as a Spin Liquid

The spin system of the Heisenberg model (s = 1/2) on a square lattice with antiferromagnetic (AFM) exchange between nearest neighbors (in which there is no long-range magnetic order at any T not equal 0) is treated as a spatially homogeneous isotropic spin liquid. The double-time temperature Green's function method is used in the framework of a second-step decoupling scheme. It is shown that, as T -> 0, the spin liquid goes over (without any change in symmetry) to a singlet state with energy (per bond) epsilon(0) = -0.352 and the correlation length diverges as xi proportional to T-1 exp(T-0/T). The spatial spin correlators oscillate in sign with distance, as in the AFM state. The theory allows one to calculate the main characteristics of the system in all temperature ranges. (C) 2002 MAIK "Nauka/Interperiodica".