Abstract

We analyze the low-temperature properties of the square-lattice quantum Heisenberg antiferromagnet, frustrated by nearest-neighbor diagonal couplings (${\mathit{J}}_{1\mathrm{\ensuremath{-}}}$${\mathit{J}}_{2}$ model), by use of Takahashi's variational spin-wave approach. Explicit formulas are obtained for the energy, specific-heat coefficient, spin-stiffness constant, uniform susceptibility, and correlation length, as well as for the static spin-spin correlators for arbitrary values of the frustration parameter \ensuremath{\alpha}=${\mathit{J}}_{2}$/${\mathit{J}}_{1}$. These results are compared with the available numerical results for 01. All quantities, mentioned above, change smoothly in the entire region 00.6. So, no special behavior is observed for 0.50.6. On the other hand, a sharp growth of the specific-heat coefficient in the region 0.60.7 is seen. In the extremely low-temperature limit, the present theory predicts a first-order phase transition from the disordered-N\'eel phase to the Ising-ordered phase, discussed by Chandra, Coleman, and Larkin.

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