Bond-operator mean field theory for the bilayer Heisenberg model

Bond-operator mean field equations for the square-lattice, S = 1/2 bilayer Heisenberg model are developed and solved numerically. In the vicinity of b oth the zero-field critical point and the field-induced transitions, compar isons are made with T = 0 and finite-temperature strong coupling expansions . The mean-field theory suggests that the quantum critical region for the f ield-induced transitions is restricted to significantly lower temperatures than one might have concluded based on strong-coupling expansions or other numerical studies.