THE BOUNDARY-CONDITION INTEGRATION TECHNIQUE - RESULTS FOR THE HUBBARD-MODEL IN 1D AND 2D

We study models of strongly correlated electrons in one and two dimensions. We exactly diagonalize small clusters with general boundary conditions (BC) and integrate over all possible BC. This technique recovers the kinetic energy part of the (extended lattice) Hamiltonian exactly in a grand-canonical formulation. A continuous range of particle densities may be described with this technique and the momentum space can be probed for arbitrary momenta. For the Hubbard Hamiltonian we recover details of the Mott-insulating behaviour for the momentum distribution function at half filling, both in 1D and 2D. Off half-tilling the shape of the canonical Fermi surface is strongly distorted in 2D with respect to the grand canonical Fermi surface. The shape of the grand canonical Fermi surface obtained by this finite-size technique reduces in the weak-coupling limit exactly to that of the infinite-lattice Fermi sea.