A first principle calculation is performed to investigate the electronic and the thermodynamic properties of a dimer in a potential characterized by the random site energy and the random screening length. The exact Hamiltonian from which the extended Hubbard model Hamiltonian is derived is...

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...

By using the irreducible double-time temperature Green functions a Dyson equation for the transverse Ising paramagnet has been derived. In the lowest order approximation the results are in agreement with one-dimensional exact results and series expansion methods.

The question of localized states in the one-dimensional Ising model with a transverse field for arbitrary boundary conditions is strictly discussed. An analytical discussion of the occurrence of acoustic and optical surface modes for finite as well as semi-infinite chaim is performed. We also...

We have proposed a new model to investigate a substitutional magnetic impurity in non-magnetic metal. The model takes into account the electron correlations in both the host narrow band and the magnetic impurity orbital. Exact solutions for the thermodynamic properties of small systems of four...

We have obtained the exact numerical results of the specific heat, the susceptibility and the correlation functions L0(T) and L1(T) for finite extended Hubbard rings at the large U limit. It is shown that the nearest neighbor interactions favor the ferromagnetic ordering. When the number of...

Exact numerical solutions are obtained for a two-site doubly degenerate Hubbard model with 2, 3 or 4 electrons to study the effect of intra-atomic exchange interaction on the ground state and the thermodynamic properties. Some properties of macroscopic systems are reproduced qualitatively. The...

The thermodynamic properties of spin-one Heisenberg ferromagnets with uniaxial and biaxial anisotropy are investigated in the molecular field approximation (MFA) and random phase approximation (RPA). In MFA a full phase diagram comprising three lines of bicritical points, is obtained.

The sensitive and complicated dependence of frustation on local structure has been clearly demonstrated with an exact numerical solution. Its connection to realistic spin-glass is discussed.

We propose a self-consistent many-body theory for the standard basis operator Green's functions and obtain an exact Dyson-type matrix equation for the interacting many-level systems. A zeroth order approximation, which neglects all the damping effects, is investigated in detail for the...