$J_1-J_2$ Quantum Heisenberg Antiferromagnet: Improved Spin-Wave Theories Versus Exact-Diagonalization Data

Abstract

We reconsider the results cocerning the extreme-quantum $S=1/2$ square-lattice Heisenberg antiferromagnet with frustrating diagonal couplings ($J_1-J_2$ model) drawn from a comparison with exact-diagonalization data. A combined approach using also some intrinsic features of the self-consistent spin-wave theory leads to the conclusion that the theory strongly overestimates the stabilizing role of quantum flutcuations in respect to the Néel phase in the extreme-quantum case $S=1/2$. On the other hand, the analysis implies that the Néel phase remains stable at least up to the limit $J_2/J_1=0.49$ which is pretty larger than some previous estimates. In addition, it is argued that the spin-wave ansatz predicts the existence of a finite range ($J_2/J_1<0.323$ in the linear spin-wave theory) where the Marshall-Peierls sigh rule survives the frustrations.

13 pages, LaTex, 7 figures on request

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