Bond-Operator Mean Field Theory for theBilayer Heisenberg Model

Abstract

Bond-operator mean field equations for the square-lattice, S=1/2 bilayer Heisenberg model are developed and solvednumerically. In the vicinity of both the zero-fieldcritical point and the field-induced transitions, comparisons are made with T=0 and finite-temperaturestrong coupling expansions.The mean-field theory suggests that the quantum criticalregion for the field-induced transitions is restricted tosignificantly lower temperatures than one might have concluded based on strong-coupling expansions or othernumerical studies.