Ground-State Phase Diagrams of the Two-Dimensional Quantum Heisenberg Spin Glass Models

Ground-state properties of the two-dimensional S=1/2 random Heisenbergmodels are investigated by the exact-diagonalization method. The phasediagram of the bond-random model (the ±J model) is the same as thatof the corresponding Ising spin glass model, in spite of quantumfluctuation. In the site-random model, the spin glass phase exists atleast in the ferro-rich case. In comparison with the correspondingIsing model, the Néel phase of this model becomes much narrower inthe ferro-rich case, while it is comparable in the antiferro-richcase.