424 research outputs found
Densities of states of the Falicov-Kimball model off half filling in infinite dimensions
An approximate analytical scheme of the dynamical mean field theory (DMFT) is
developed for the description of the electron (ion) lattice systems with
Hubbard correlations within the asymmetric Hubbard model where the chemical
potentials and electron transfer parameters depend on an electron spin (a sort
of ions). Considering a complexity of the problem we test the approximation in
the limiting case of the infinite- spinless Falicov-Kimball model. Despite
the fact that the Falicov-Kimball model can be solved exactly within DMFT, the
densities of states of localized particles have not been completely
investigated off half filling. We use the approximation to obtain the spectra
of localized particles for various particle concentrations (chemical
potentials) and temperatures. The effect of a phase separation phenomenon on
the spectral function is considered.Comment: 9 pages, 11 figures, submitted to Phys. Rev.
Phase Transitions in the Pseudospin-Electron Model
A review of the present state of investigations of the pseudospin-electron
model (PEM), which is used in the theory of strongly correlated electron
systems, is given. The model is used to describe the systems with the locally
anharmonic elements of structure represented in the model by pseudospins. The
consideration is based on the dynamical mean field theory approach and the
generalized random phase approximation. Electron spectrum and thermodynamics of
the model are investigated; the cases of the simplified model, the model with
strong interaction and the two-sublattice model are studied more in detail. The
phase transitions into other uniform or modulated states as well as
superconducting phases are described; the criteria of their realization are
established. Based on this, the description of structural and dielectric
(ferroelectric type) instabilities, phase separation and bistability phenomena
is given. A comparison is made with the thermodynamics of the Falicov-Kimball
model (which can be considered as a particular case of PEM). The possibility of
applying the PEM to the analysis of thermodynamics of the real HTSC systems is
discussed. Attention is paid to the unsolved problems in the study of PEM
Energy spectrum and phase diagrams of two-sublattice hard-core boson model
The energy spectrum, spectral density and phase diagrams have been obtained
for two-sublattice hard-core boson model in frames of random phase
approximation approach. Reconstruction of boson spectrum at the change of
temperature, chemical potential and energy difference between local positions
in sublattices is studied. The phase diagrams illustrating the regions of
existence of a normal phase which can be close to Mott-insulator (MI) or
charge-density (CDW) phases as well as the phase with the Bose-Einstein
condensate (SF phase) are built.Comment: 9 pages, 4 figure
Some Peculiarities of Proton Transport in Quasi-One-Dimensional Hydrogen-Bonded Chains
The protonic conductivity in the hydrogen bonded chains is investigated
theoretically in the framework of the two-stage transport model. The strong
interactions with optical phonon stretching mode are considered. We obtain the
transition from the insulator to the metal-type state from the temperature
dependencies of the hopping conductivity and analyze the influence of the
specific Grotthuss mechanism on the transition character. We investigate also
the main peculiarities in the frequency dependencies of the band and hopping
conductivity parts which appear due to above-mentioned two-stage process of the
proton migration along the chain.Comment: 21 pages, RevTeX, 9 eps figure
Phase separation in lithium intercalated anatase: A theory
Lithium intercalated anatase used in Li-ion batteries has some special
features: coexistence of Li-rich and Li-poor phases as well as two possible
positions for Li ions in the oxygen tetrahedron. A theoretical description of
the compound considering those peculiarities is presented. As shown by the
performed symmetry analysis, the intercalation induced lattice deformation can
be accompanied by the ordering of antiferroelectric type (internal
piezoeffect). In the following step, a qualitative illustration of the phase
separation in the lithiated anatase is given within the Landau expansion at the
proper choice of coefficients. A microscopic model for description of the
compound is also proposed which combines features of the Mitsui and
Blume-Emery-Griffits models and utilizes the symmetry analysis results. Various
ground state and temperature-dependent phase diagrams of the model are studied
to find a set of model parameters corresponding to the lithiated anatase. A
phase separation into the empty and half-filled phases in a wide temperature
range has been found closely resembling the phase coexistence in the
intercalated crystal. In the framework of the model, the two-position Li
subsystem could have the ordering of ferro- or antiferroelectric types which,
however, has not been yet observed by the experiment.Comment: 19 pages, Reported at the Conference "Statistical Physics: Modern
Trends and Applications" dedicated to the 100-th anniversary of N.N.
Bogolyubov (June 23--25, 2009 Lviv, Ukraine
Phase diagrams of the Bose-Hubbard model at finite temperature
The phase transitions in the Bose-Hubbard model are investigated. A
single-particle Green's function is calculated in the random phase
approximation and the formalism of the Hubbard operators is used. The regions
of existence of the superfluid and Mott insulator phases are established and
the (the chemical potential -- transfer parameter) phase diagrams are
built. The influence of temperature change on this transition is analyzed and
the phase diagram in the plane is constructed. The role of thermal
activation of the ion hopping is investigated by taking into account the
temperature dependence of the transfer parameter. The reconstruction of the
Mott-insulator lobes due to this effect is analyzed
Mott transition in the asymmetric Hubbard model at half-filling within dynamical mean-field theory
We apply the approximate analytic methods to the investigation of the band
structure of the asymmetric Hubbard model where the chemical potentials and
electron transfer parameters depend on the electron spin (type of
quasiparticles). The Hubbard-I and alloy-analogy approximations are the
simplest approximations which are used. Within the alloy-analogy approximation,
the energy band of particles does not depend on the transfer parameter of
particles of another sort. It means that the gap in the spectrum opens at the
critical value that is the same in two different limiting cases: the
Falicov-Kimball model and the standard Hubbard model. The approximate analytic
scheme of the dynamical mean-field theory is developed to include into the
theory the scattering of particles responsible for the additional mechanism
(due to the transfer of particles of another sort) of the band formation. We
use the so-called GH3 approach that is a generalization of the Hubbard-III
approximation. The approach describes the continuous Mott transition with the
value dependent on a ratio of transfer parameters of different
particles.Comment: 10 pages, 10 figure
Bose-Einstein condensation and/or modulation of "displacements" in the two-state Bose-Hubbard model
Instabilities resulting in Bose-Einstein condensation and/or modulation of
"displacements" in a system of quantum particles described by a two-state
Bose-Hubbard model (with an allowance for the interaction between particle
displacements on different lattice sites) are investigated. A possibility of
modulation, which doubles the lattice constant, as well as the uniform
displacement of particles from equilibrium positions are studied. Conditions
for realization of the mentioned instabilities and phase transitions into the
SF phase and into the "ordered" phase with frozen displacements are analyzed.
The behaviour of order parameters is investigated and phase diagrams of the
system are calculated both analytically (ground state) and numerically (at
non-zero temperatures). It is revealed that the SF phase can appear as an
intermediate one between the normal and "ordered" phases, while a supersolid
phase is thermodynamically unstable and does not appear. The relation of the
obtained results to the lattices with the double-well local potentials is
discussed.Comment: 17 pages, 11 figure
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