Staggered magnetisation in the Heisenberg antiferromagnet

In the presence of a staggered magnetic field, the plaquette expansion of the Lanczos matrix elements are obtained for the antiferromagnetic 2 D Heisenberg model up to order 1/Np (Np is the number of plaquettes on the lattice). The resulting approximated tri-diagonal form of the Hamiltonian is diagonalised for various values of the field strength in the Np → ∞ limit for the ground state energy density. From the behaviour of the ground-state energy density at weak fields, the staggered magnetisation at this order in the plaquette expansion was calculated. 10 refs., 4 figs