1ST-ORDER TRANSITION IN FRUSTRATED QUANTUM ANTIFERROMAGNETS

A two-dimensional frustrated quantum Heisenberg model with first-, second- (diagonal), and third-neighbor couplings (J1-J2-J3 Model) is studied near the phase boundary of the antiferromagnetic (pi,pi) phase. It is shown that quantum corrections remove accidental classical degeneracy and ensure a nonzero hysteresis width for the J3 = 0 transition between commensurate (pi,pi) and (pi,0) phases, so that finite J3 is necessary to produce a continuous transition through the intermediate disordered phase.