Charge ordering and superconductivity in bipolaronic systems with diagonal disorder

The phase diagram of the extended Hubbard model with intraatomic attraction and random site energy is studied for the half-filled band case. In the strong coupling limit the problem is transformed to the ansiotropic Heisenberg model in an uniaxial random field, and then analyzed in the mean field approximation. Two types of random site energy distribution are considered: the two-delta and the square distribution. It is found that the diagonal disorder strongly affects both the charge ordering (CO) and the singlet superconductivity (SS), and leads to quite interesting multicritical behavior including lines of bicritical points, tricritical points and critical end points, as well as a new multicritical point. Increasing disorder can induce a CO to SS transition in a wide range of parameter values. Owing to the formal equivalence of the effective Hamiltonians in the strong coupling limit, the results are applicable to both the on-site and the intersite bipolaronic systems.