Quantum-classical escape-rate transition of a biaxial spin system with a longitudinal field: A perturbative approach

The quantum-classical transition of the escape rate of the spin model H = -DSz2 - HzSz + BSx2 is investigated by a perturbative approach with respect to B [D. A. Garanin, J. Phys A 24, L61 (1991)]. The transition is first order for B < B-c(H-z), the boundary line going to zero as B-c/D similar to 1 - H-z/(2SD) in the strongly biased limit. The range of the first-order transition is thus larger than for the model H = -DSz2 - HzSz - HxSx studied earlier, where in the strongly biased case H-xc/(2SD)similar to[1 - H-z/(2SD)](3/2). The temperature of the quantum-classical transition, T-0, behaves linearly in the strongly biased case for both models: T(0)similar to 2SD-H-z. [S0163-1829(99)02205-5].