Em. Chudnovsky et Da. Garanin, FIRST-ORDER AND 2ND-ORDER TRANSITIONS BETWEEN QUANTUM AND CLASSICAL REGIMES FOR THE ESCAPE RATE OF A SPIN SYSTEM, Physical review letters, 79(22), 1997, pp. 4469-4472
We have found a novel feature of the bistable large-spin model describ
ed by the Hamiltonian H = -DS2 -HxSx. The crossover from thermal to qu
antum regime for the escape rate can be either first (H-x < SD/2) or s
econd (SD/2 < H-x < 2SD) order, that is, sharp or smooth, depending on
the strength of the transverse field. This prediction can be tested e
xperimentally in molecular magnets like Mn12Ac.