The Phase Diagram of Correlated Electrons in a Lattice of Berry   Molecules
  A model for correlated electrons in a lattice with local additional spin--1 degrees of freedom inducing constrained hopping, is studied both in the low density limit and at quarter filling. We show that in both 1D and 2D two particles form a bound state even in presence of a repulsive U<U_c. A picture of a dilute Bose gas, leading to off-diagonal long range order (LRO) in 2D (quasi-LRO in 1D), is supported by quantitative calculations in 1D which allow for a determination of the phase diagram. 
Giuseppe Santoro, Marco Airoldi, Nicola Manini, Erio Tosatti, Alberto   Parola