From independent particle towards collective motion in two electron   square lattices
  The two dimensional crossover from independent particle towards collective motion is studied using 2 spinless fermions interacting via a U/r Coulomb repulsion in a LxL square lattice with periodic boundary conditions and nearest neighbor hopping t. Three regimes characterize the ground state when U/t increases. Firstly, when the fluctuation $\Delta r$ of the spacing r between the two particles is larger than the lattice spacing, there is a scaling length $Lo=\sqrt{8}\pi^2(t/U)$ such that the relative fluctuation $\Delta r/<r>$ is a universal function of the dimensionless ratio L/Lo, up to finite size corrections of order $L^{-2}$. L<Lo and L>Lo are respectively the limits of the free particle Fermi motion and of the correlated motion of a Wigner molecule. Secondly, when U/t exceeds a threshold U*(L)/t, $\Delta r$ becomes smalller than the lattice spacing, giving rise to a correlated lattice regime where the previous scaling breaks down and analytical expansions in powers of (t/U) become valid. A weak random potential reduces the scaling length and favors the correlated motion. 
Moises Martinez and Jean-Louis Pichard (CEA/DSM/SPEC, Centre d'Etudes   de Saclay, France)