Criticality and Mott-glass phase in a disordered 2D quantum spin systems

We use quantum Monte Carlo simulations to study a disordered S=1/2 Heisenberg quantum spin model with three different nearest-neighbor interactions, J1<=J2<=J3, on the square lattice. We consider the regime in which J1 represents weak bonds, and J2 and J3 correspond to two kinds of stronger bonds (dimers) which are randomly distributed on columns forming coupled 2-leg ladders. When increasing the average intra-dimer coupling (J2+J3)/2, the system undergoes a Neel to quantum glass transition of the ground state and later a second transition into a quantum paramagnet. The quantum glass phase is of the gapless Mott glass type (i.e., in boson language it is incompressible at temperature T = 0), and we find that the temperature dependence of the uniform magnetic susceptibility follows the stretched exponential form x~exp(-b/T^alpha) with 0 < alpha < 1. At the Neel-glass transition we observe the standard O(3) critical exponents, which implies that the Harris criterion for the relevance of the disorder is violated in this system.