GRIFFITHS SINGULARITIES IN THE DISORDERED PHASE OF A QUANTUM ISING SPIN-GLASS

We study a model for a quantum Ising spin glass in two space dimension s by Monte Carlo simulations. In the disordered phase al T=0, rare str ongly correlated regions give rise to strong Griffiths singularities, as originally found by McCoy for a one-dimensional model. We iind that there are power law distributions of the local susceptibility and loc al nonlinear susceptibility, which are characterized by a smoothly var ying dynamical exponent z. Over a lange of the disordered phase near t he quantum transition, the local nonlinear susceptibility diverges. Th e local susceptibility does not diverge in the disordered phase but do es diverge at the critical point. Approaching the critical point from the disordered phase, the limiting value of z seems to equal its Value precisely at criticality,even though the physics of these two cases s eems rather different.