QUANTUM FLUCTUATIONS IN A DISORDERED 2-DIMENSIONAL SPIN MODEL

We study the effect of quantum fluctuations on the spin stiffness of a disordered two-dimensional anisotropic spin model within an 1/S expan sion. We find that these fluctuations, very weak in the pure system, m ay be quite strong in the presence of bond disorder provided the latte r introduces frustration in the system. As a consequence of the disord er-induced increase in the amplitude of zero-point fluctuations, the s pin stiffness constant of the system vanishes in certain regions of pa rameter space, leading to the appearance of a spin-liquid phase in par ts of the phase diagram where a spin-glass phase would be expected cla ssically.