PLAQUETTE EXPANSION OF THE 2-DIMENSIONAL ANTIFERROMAGNETIC HEISENBERG-MODEL

The plaquette expansion of the Lanczos recursion method is applied to the two-dimensional antiferromagnetic Heisenberg model. Connected Hamiltonian moments [H(n)]c are calculated with respect to the Neel state up to n = 6. The subsequent plaquette expansion of the Lanczos matrix in the number of plaquettes on the lattice, N(p), is determined to order 1/N(p). Diagonalizing the Lanczos matrix in this form gives a value of the energy density of -0.664 in the limit N(p) --> infinity, in good agreement with existing calculations.