Finite-Temperature Phase Transition in the Montorsi-Rasetti Model.

PACS 05.30F- Fermion systems and electron gas. PACS 71.10- General theories and computational techniques. Abstract.- Exact diagonalization and quantum Monte Carlo methods are used to compute the specific heat and the charge and spin correlation functions for a two-dimensional system of correlated electrons, described by the Montorsi-Rasetti model. Our results strongly suggest the occurrence of an unusual phase transition at finite temperatures. Recently, Montorsi and Rasetti (MR) introduced a modified Hubbard model [1-4]. The MR model describes a system of electrons which hop to nearest-neighbour lattice sites and which are coupled through a Coulomb interaction when two electrons of opposite spin occupy the same lattice site. Unlike in the Hubbard model, the nearest-neighbour hopping is occu-pation-dependent and does not conserve the spin. The Hamiltonian for the MR model is obtained from the Hamiltonian for the Hubbard model by including spin-flip hopping processes and hopping processes depending on the relative site occupation. Physically the former may be due to the presence of spin-orbit coupling [5] and the latter to the presence of a bond-charge interaction [6-9]. The MR Hamiltonian read