FIELD-INDUCED CROSSOVER-BEHAVIOR IN QUANTUM HEISENBERG SPIN-GLASSES WITH RANDOM-ANISOTROPY AXES

The infinite-range quantum Heisenberg spin-glass model with random-anisotropy axes and external magnetic field (h) is studied by means of the thermofield dynamics for the spin value S = 1/2. The stability of the mean-field-type solution against the action of fluctuations has been evaluated in the presence of h leading to the upper and lower critical lines in the field-temperature (h-T) plane. For small values of the anisotropy variance we find a crossover of the upper critical line from the de Almeida-Thouless-type behavior for small fields to the Gabay-Toulouse-like behavior for large fields and a reversed type of behavior at the lower critical line for corresponding values of the random anisotropy-in close analogy to the recently analyzed quantum vector spin glass with random Dzyaloshinsky-Moriya interactions.