For the Keldysh-Kopaev and the Mattis-Langer models the complete phase diagram (T,μ) is characterized. The degree of criticality of the electron-hole pairing fluctuation operator is obtained as a function of the degree of doping.

The model of Mattis and Langer for superconductors with a structural distortion associated with doubling of the lattice periodicity is studied. New results for the zero-temperature superconducting gap are found in the case of more than a half-filled band. The modification of both the electron...

Two three-dimensional models exhibiting coexistence of superconductivity and lattice distortion are discussed. One of the models, studied in detail by Rusinov, Do Chan Kat and Kopayev, takes into account the superconductivity inter- and intra-band pairing phenomenologically. The other has been...

An exactly solvable Vonsovsky-Zener type model for a superconductor with magnetic impurities is analyzed. The BCS pairing cut-off and the general spin number of the magnetic ions are taken into account.

A lattice model with full rotational symmetry is discussed for describing nematic liquid crystals. It is a lattice-gas approximation of continuum liquid with dispersion forces between long centrosymmetric molecules. Orientational long-range order at low temperatures and large chemical potentials...

A method for solving the finite-volume Kirkwood-type correlation equations for tempered boundary conditions is developed. The central idea is an analytic continuation in the activity of the resolvent formulas for the solutions. The uniqueness theorem is proved for activities in a larger domain...

The main theoren about the exactness of the generalized version of the Bogoliubov (Jr.) thermodynamically-equivalent Hamiltonian method for Dicke-type models which include some hidden variables, is proved. Some generalizations are done of earlier investigations according to a Dicke-type model...

We present a simple exactly soluble model which manifests two kinds of Bose–Einstein condensations. They occur in two stages: for intermediate densities (negative chemical potentials μ0<μ<0) one has a non-conventional Bose condensation in the mode k=0 due to a diagonal perturbation of the perfect Bose-gas (PBG), whereas for μ=0 and large densities ρ>ρc0(θ) one gets a conventional (generalized) Bose-condensation corresponding to the standard mechanism of...</μ<0)>

The thermodynamic properties of a generalized Dicke model including (i) spatial variation of the electromagnetic field, (ii) classical Debye vibrations of the atoms around their equilibrium positions, are investigated in the thermodynamic limit in an exact fashion. Contrary to the ordinary case...

Two mean-field models describing the metal-insulator phase transition are studied. In the model due to Keldysh and Kopaev (1964) different effective masses in the valence and conduction bands are considered and in the model due to Mattis and Langer (1970) the exact s.c. and b.c.c. densities of...