GROUND-STATE PHASE-DIAGRAMS OF THE 2-DIMENSIONAL QUANTUM HEISENBERG SPIN-GLASS MODELS

Ground-state properties of the two-dimensional S=1/2 random Heisenberg models are investigated by the exact-diagonalization method. The phase diagram of the bond-random model (the +/- J model) is the same as that of the corresponding Ising spin glass model, in spite of quantum fluctuation. In the site-random model, the spin glass phase exists at least in the ferro-rich case. In comparison with the corresponding Ising model, the Neel phase of this model becomes much narrower in the ferro-rich case, while it is comparable in the antiferro-rich case.