SPIN-PEIERLS DIMERIZATION AND FRUSTRATION IN 2-DIMENSIONAL ANTIFERROMAGNETS

We have investigated the stability of the antiferromagnetic order in the ground state of the two-dimensional Heisenberg model against a lattice dimerization produced by the electron-lattice interaction. Unlike the one-dimensional case, we found that a critical coupling is required to form a dimerized state via a first-order phase transition. We have also considered the combined effects of frustrating second-neighbour interactions and lattice dimerization in destroying the Neel order. Our approach is based on the Schwinger representation of spin operators, and is in excellent quantitative agreement with exact numerical results on finite lattices.