Physical Review B 82,17 (2010)
We use quantum Monte Carlo simulations to study effects of disorder on the quantum phase transition occurring versus the ratio g = J/J' in square-lattice dimerized S = 1/2 Heisenberg antiferromagnets with intradimer and interdimer couplings J and J'. The dimers are either randomly distributed (as in the classical dimer model), or come in parallel pairs with horizontal or vertical orientation. In both cases the transition violates the Harris criterion, according to which the correlation-length exponent should satisfy v >= 1. We do not detect any deviations from the three-dimensional O(3) universality class obtaining in the absence of disorder (where v approximate to 0.71). We discuss special circumstances which allow v < 1 for the type of disorder considered here.
Date of this Version