J(1)-J(2) QUANTUM HEISENBERG-ANTIFERROMAGNET - IMPROVED SPIN-WAVE THEORIES VERSUS EXACT-DIAGONALIZATION DATA

We reconsider the results concerning the extreme-quantum S = 1/2 squar e-lattice Heisenberg antiferromagnet with frustrating diagonal couplin gs (the J1-J2 model) drawn from a comparison with exact-diagonalizatio n data. A combined approach, also using some intrinsic features of the self-consistent spin-wave theory, leads to the conclusion that the th eory strongly overestimates the stabilizing role of quantum fluctuatio ns with respect to the Neel phase in the extreme-quantum case S = 1/2. On the other hand, the analysis implies that the Neel phase remains s table at least up to the limit J2/J1 = 0.49, which is larger than some previous estimates. In addition, it is argued that the spin-wave ansa tz predicts the existence of a finite range (J2/J1 < 0.323 in linear s pin-wave theory) where the Marshall-Peierls sip rule survives the frus trations.