PHASE-TRANSITIONS IN THE RANDOM-FIELD ISING-MODEL IN THE PRESENCE OF A TRANSVERSE FIELD

We have studied the phase transition behaviour of the random field Isi ng model in the presence of a transverse (or tunnelling) field. The me an field phase diagram has been studied in detail, and in particular t he nature of the transition induced by the tunnelling (transverse) fie ld at zero temperature. A modified hyperscaling relation for the zero- temperature transition has been derived using the Suzuki-Trotter forma lism and a modified 'Harris criterion'. A mapping of the model to a ra ndomly diluted antiferromagnetic Ising model in uniform longitudinal a nd transverse field is also given.