152 research outputs found
Dependence of transport coefficients of Yb(RhCo)Si intermetallics on temperature and cobalt concentration
Dependence of transport coefficients of the Yb(RhCo)Si
series of alloys on temperature and cobalt concentration is explained by an
asymmetric Anderson model which takes into account the exchange scattering of
conduction electrons on ytterbium ions and the splitting of 4-states by the
crystalline electric field (CEF). The substitution of rhodium by cobalt is
described as an increase of chemical pressure which reduces the exchange
coupling and the CEF splitting. The scaling analysis and numerical NCA solution
of the model show that the effective degeneracy of the 4-state at a given
temperature depends on the relative magnitude of the Kondo scale and the CEF
splitting. Thus, we find that dependence of the thermopower, , on
temperature and cobalt concentration can be understood as an interplay of
quantum fluctuations, driven by the Kondo effect, and thermal fluctuations,
which favor a uniform occupation of the CEF states. The theoretical model
captures all the qualitative features of the experimental data and it explains
the evolution of the shape of with the increase of cobalt concentration.Comment: 8 pages, 4 figure
Hartree-Fock study of electronic ferroelectricity in the Falicov-Kimball model with - hopping
The Hartree-Fock (HF) approximation with the charge-density-wave (CDW)
instability is used to study the ground-state phase diagram of the spinless
Falicov-Kimball model (FKM) extended by - hopping in two and three
dimensions. It is shown that the HF solutions with the CDW instability
reproduce perfectly the two-dimensional intermediate coupling phase diagram of
the FKM model with - hopping calculated recently by constrained path
Monte Carlo (CPMC) method. Using this fact we have extended our HF study on
cases that have been not described by CPMC, and namely, (i) the case of small
values of -electron hopping integrals, (ii) the case of weak Coulomb
interactions and (iii) the three-dimensional case. We have found that
ferroelectricity remains robust with respect to the reducing strength of
coupling (-electron hopping) as well as with respect to the increasing
dimension of the system.Comment: 13 pages, 5 figure
F-electron spectral function of the Falicov-Kimball model in infinite dimensions: the half-filled case
The f-electron spectral function of the Falicov-Kimball model is calculated
via a Keldysh-based many-body formalism originally developed by Brandt and
Urbanek. We provide results for both the Bethe lattice and the hypercubic
lattice at half filling. Since the numerical computations are quite sensitive
to the discretization along the Kadanoff-Baym contour and to the maximum cutoff
in time that is employed, we analyze the accuracy of the results using a
variety of different moment sum-rules and spectral formulas. We find that the
f-electron spectral function has interesting temperature dependence becoming a
narrow single-peaked function for small U and developing a gap, with two
broader peaks for large U.Comment: (13 pages, 11 figures, typeset in RevTex 4
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
Perturbation expansion for 2-D Hubbard model
We develop an efficient method to calculate the third-order corrections to
the self-energy of the hole-doped two-dimensional Hubbard model in space-time
representation. Using the Dyson equation we evaluate the renormalized spectral
function in various parts of the Brillouin zone and find significant
modifications with respect to the second-order theory even for rather small
values of the coupling constant U. The spectral function becomes unphysical for
, where W is the half-width of the conduction band. Close to the
Fermi surface and for U<W, the single-particle spectral weight is reduced in a
finite energy interval around the Fermi energy. The increase of U opens a gap
between the occupied and unoccupied parts of the spectral function.Comment: 17 pages, 11 Postscript figures, Phys. Rev. B, accepte
Slave boson theory of the extended Falicov-Kimball model
The extended Falicov-Kimball model, with both an on-site hybridization
potential and dispersive narrow band, is examined within the saddle-point
approximation to the Kotliar-Ruckenstein slave boson theory. We first set the
hybridization potential to zero and find that the phase diagram depends
strongly upon the orbital structure: for degenerate orbitals, a
correlated-insulating state is found at sufficiently strong interaction
strengths, whereas a finite orbital energy difference can lead to discontinuous
valence transitions. The obtained phase diagram is very sensitive to the
presence of a finite hybridization potential. As in Hartree-Fock theory, we
find an enhancement of the hybridization by the inter-orbital Coulomb
repulsion. The more precise treatment of correlation effects, however, leads to
large deviations from the Hartree-Fock results. In the limit of vanishing
hybridization an excitonic insulator state is only found when the orbitals are
degenerate, which restricts this phase to a much smaller parameter space than
in other available mean-field theories.Comment: 23 pages, 10 figure
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.
Thermoelectric effects in correlated quantum dots and molecules
We investigate thermoelectric properties of correlated quantum dots and
molecules, described by a single level Anderson model coupled to conduction
electron leads, by using Wilson's numerical renormalization group method. In
the Kondo regime, the thermopower, , exhibits two sign changes, at
temperatures and . We find that is of order
the level width and , where is the
position of the Kondo induced peak in the thermopower and is the Kondo
scale. No sign change is found outside the Kondo regime, or, for weak
correlations, making a sign change in a particularly sensitive signature
of strong correlations and Kondo physics. For molecules, we investigate the
effect of screening by conduction electrons on the thermoelectric transport. We
find that a large screening interaction enhances the figure of merit in the
Kondo and mixed valence regimes.Comment: 4 pages, 3 figures; to appear in the Proceedings of the International
Conference on Strongly Correlated Electron Systems, Santa Fe 2010; revised
version: typos corrected and references update
- …