Finite temperature behavior of strongly disordered quantum magnets coupled to adissipative bath**Dedicated to Professor Thomas Nattermann on the occasion of his 60th anniversary.

Abstract

We study the effect of dissipation on the infinite randomness fixed point and theGriffiths–McCoy singularities of random transverse Ising systems in chains, ladders and intwo dimensions. A strong disorder renormalization group scheme is presentedthat allows the computation of the finite temperature behavior of the magneticsusceptibility and the spin specific heat. In the case of ohmic dissipation thesusceptibility displays a crossover from Griffiths–McCoy behavior (with a continuouslyvarying dynamical exponent) to classical Curie behavior at some temperatureT*. The specific heat displays Griffiths–McCoy singularities over the whole temperaturerange. For super-ohmic dissipation we find an infinite randomness fixed point within thesame universality class as the transverse Ising system without dissipation. In this case thephase diagram and the parameter dependence of the dynamical exponent in theGriffiths–McCoy phase can be determined analytically.