60 research outputs found
Nonresonant Raman and inelastic X-ray scattering in the charge-density-wave phase of the spinless Falicov-Kimball model
Nonresonant inelastic light and X-ray scattering is investigated for the
spinless Falicov-Kimball model on an infinite-dimensional hypercubic lattice
with a charge-density-wave phase at half filling. The many-body density of
states (DOS) is found for different values of the Coulomb repulsion ,
ranging from a dirty metal to a Mott insulator. At zero temperature, the charge
gap is exactly equal to ; increasing the temperature rapidly fills the gap
with subgap states. The nonresonant response function for Raman and inelastic
X-ray scattering shows peaks connected with transitions over the gap and
transitions that involve subgap states. In the case of X-ray scattering (when
both energy and momentum are transferred), the response function illustrates
features of dynamical screening (vertex corrections) in the different
(nonresonant) symmetry channels ( and ). We also derive
and verify the first moment sum rules for the (nonresonant) Raman and inelastic
X-ray response functions.Comment: 19 pages, 17 figure
Hydrogen transport in superionic system Rb3H(SeO4)2: a revised cooperative migration mechanism
We performed density functional studies of electronic properties and
mechanisms of hydrogen transport in Rb3H(SeO4)2 crystal which represents
technologically promising class M3H(XO4)2 of proton conductors (M=Rb,Cs, NH4;
X=S,Se). The electronic structure calculations show a decisive role of lattice
dynamics in the process of proton migration. In the obtained revised mechanism
of proton transport, the strong displacements of the vertex oxygens play a key
role in the establishing the continuous hydrogen transport and in the achieving
low activation energies of proton conduction which is in contrast to the
standard two-stage Grotthuss mechanism of proton transport. Consequently, any
realistic model description of proton transport should inevitably involve the
interactions with the sublattice of the XO4 groups.Comment: 11 pages, 11 figures, to appear in Physical Review
Phase transitions and quantum effects in anharmonic crystals
The most important recent results in the theory of phase transitions and
quantum effects in quantum anharmonic crystals are presented and discussed. In
particular, necessary and sufficient conditions for a phase transition to occur
at some temperature are given in the form of simple inequalities involving the
interaction strength and the parameters describing a single oscillator. The
main characteristic feature of the theory is that both mentioned phenomena are
described in one and the same setting, in which thermodynamic phases of the
model appear as probability measures on path spaces. Then the possibility of a
phase transition to occur is related to the existence of multiple phases at the
same values of the relevant parameters. Other definitions of phase transitions,
based on the non-differentiability of the free energy density and on the
appearance of ordering, are also discussed
Phase Separation and Charge-Ordered Phases of the d = 3 Falicov-Kimball Model at T>0: Temperature-Density-Chemical Potential Global Phase Diagram from Renormalization-Group Theory
The global phase diagram of the spinless Falicov-Kimball model in d = 3
spatial dimensions is obtained by renormalization-group theory. This global
phase diagram exhibits five distinct phases. Four of these phases are
charge-ordered (CO) phases, in which the system forms two sublattices with
different electron densities. The CO phases occur at and near half filling of
the conduction electrons for the entire range of localized electron densities.
The phase boundaries are second order, except for the intermediate and large
interaction regimes, where a first-order phase boundary occurs in the central
region of the phase diagram, resulting in phase coexistence at and near half
filling of both localized and conduction electrons. These two-phase or
three-phase coexistence regions are between different charge-ordered phases,
between charge-ordered and disordered phases, and between dense and dilute
disordered phases. The second-order phase boundaries terminate on the
first-order phase transitions via critical endpoints and double critical
endpoints. The first-order phase boundary is delimited by critical points. The
cross-sections of the global phase diagram with respect to the chemical
potentials and densities of the localized and conduction electrons, at all
representative interactions strengths, hopping strengths, and temperatures, are
calculated and exhibit ten distinct topologies.Comment: Calculated density phase diagrams. Added discussions and references.
14 pages, 9 figures, 4 table
Strong-coupling approach for strongly correlated electron systems
A perturbation theory scheme in terms of electron hopping, which is based on
the Wick theorem for Hubbard operators, is developed. Diagrammatic series
contain single-site vertices connected by hopping lines and it is shown that
for each vertex the problem splits into the subspaces with ``vacuum states''
determined by the diagonal Hubbard operators and only excitations around these
vacuum states are allowed. The rules to construct diagrams are proposed. In the
limit of infinite spatial dimensions the total auxiliary single-site problem
exactly splits into subspaces that allows to build an analytical
thermodynamically consistent approach for a Hubbard model. Some analytical
results are given for the simple approximations when the two-pole
(alloy-analogy solution) and four-pole (Hartree-Fock approximation) structure
for Green's function is obtained. Two poles describe contribution from the
Fermi-liquid component, which is dominant for small electron and hole
concentrations (``overdoped case'' of high-'s), whereas other two describe
contribution from the non-Fermi liquid and are dominant close to half-filling
(``underdoped case'').Comment: 14 pages, revtex, feynmf, 5 EPS figures, two-column PRB style,
published in PR
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