Plaquette expansion of the two-dimensional antiferromagnetic Heisenberg model

Abstract

The plaquette expansion of the Lanczos recursion method is applied to the two-dimensional antiferromagnetic Heisenberg model. Connected Hamiltonian moments Hnc are calculated with respect to the Néel state up to n=6. The subsequent plaquette expansion of the Lanczos matrix in the number of plaquettes on the lattice, Np, is determined to order 1/Np. Diagonalizing the Lanczos matrix in this form gives a value of the energy density of -0.664 in the limit Np→, in good agreement with existing calculations. © 1994 The American Physical Society.