QUANTUM PHASE-TRANSITIONS IN A 3-DIMENSIONAL FRUSTRATED S=1 2 HEISENBERG-ANTIFERROMAGNET/

The spherically symmetric Green's function approximation it; used to s tudy a three-dimensional (3D) S=1/2 antiferromagnet described by isotr opic Heisenberg Hamiltonian. Detailed numerical calculations are carri ed out for the simple cubic lattice with antiferromagnetic nearest- an d next-nearest-neighbor exchange interactions (J(1)-J(2), model). We o btained the phase diagram indicating two regions of temperature and fr ustration parameters p=J(2)/(J(1)+J(2)) where two different ordered st ates are possible. The absence of the long-range order (LRO) in 3D lat tice even at zero temperature at some values of frustration is emphasi zed. At zero temperature with increasing frustration the system endure s the second-order phase transitions from the Neel-type LRO state to t he stripe-type LRO state through the spin liquid state.