FRUSTRATED 2-DIMENSIONAL QUANTUM HEISENBERG-ANTIFERROMAGNET AT LOW-TEMPERATURES

We analyze the low-temperature properties of the square-lattice quantum Heisenberg antiferromagnet, frustrated by nearest-neighbor diagonal couplings (J1-J2 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 alpha = J2/J1. These results are compared with the available numerical results for 0 < alpha < 1. All quantities, mentioned above, change smoothly in the entire region 0 < alpha < 0.6. So, no special behavior is observed for 0.5 < alpha < 0.6. On the other hand, a sharp growth of the specific-heat coefficient in the region 0.6 < alpha < 0.7 is seen. In the extremely low-temperature limit, the present theory predicts a first-order phase transition from the disordered-Neel phase to the Ising-ordered phase, discussed by Chandra, Coleman, and Larkin.