61 research outputs found
Gap ratio in anharmonic charge-density-wave systems
Many experimental systems exist that possess charge-density-wave order in
their ground state. While this order should be able to be described with models
similar to those used for superconductivity, nearly all systems have a ratio of
the charge-density-wave order parameter to the transition temperature that is
too high for conventional theories. Recent work explained how this can happen
in harmonic systems, but when the lattice distortion gets large, anharmonic
effects must play an increasingly important role. Here we study the gap ratio
for anharmonic charge-density wave systems to see whether the low-temperature
properties possess universality as was seen previously in the transition
temperature and to see whether the explanation for the large gap ratios
survives for anharmonic systems as well.Comment: (5 pages, 3 figures, ReVTeX
Semiclassical approach to calculating the influence of local lattice fluctuations on electronic properties of metals
We propose a new semiclassical approach based on the dynamical mean field
theory to treat the interactions of electrons with local lattice fluctuations.
In this approach the classical (static) phonon modes are treated exactly
whereas the quantum (dynamical) modes are expanded to second order and give
rise to an effective semiclassical potential. We determine the limits of
validity of the approximation, and demonstrate its usefulness by calculating
the temperature dependent resistivity in the Fermi liquid to polaron crossover
regime (leading to `saturation behavior') and also isotope effects on
electronic properties including the spectral function, resistivity, and optical
conductivity, problems beyond the scope of conventional diagrammatic
perturbation theories.Comment: 11 pages, 7 figure
Semiclassical action based on dynamical mean-field theory describing electrons interacting with local lattice fluctuations
We extend a recently introduced semiclassical approach to calculating the
influence of local lattice fluctuations on electronic properties of metals and
metallic molecular crystals. The effective action of electrons in degenerate
orbital states coupling to Jahn-Teller distortions is derived, employing
dynamical mean-field theory and adiabatic expansions. We improve on previous
numerical treatments of the semiclassical action and present for the
simplifying Holstein model results for the finite temperature optical
conductivity at electron-phonon coupling strengths from weak to strong.
Significant transfer of spectral weight from high to low frequencies is
obtained on isotope substitution in the Fermi-liquid to polaron crossover
regime.Comment: 10 pages, 7 figure
Reentrant charge order transition in the extended Hubbard model
We study the extended Hubbard model with both on-site and nearest neighbor
Coulomb repulsion ( and , respectively) in the Dynamical Mean Field
theory. At quarter filling, the model shows a transition to a charge ordered
phase with different sublattice occupancies n_A \nen_B. The effective mass
increases drastically at the critical and a pseudo-gap opens in the
single-particle spectral function for higher values of . The -curve
has a negative slope for small temperatures, i.e. the charge ordering
transition can be driven by increasing the temperature. This is due to the
higher spin-entropy of the charge ordered phase.Comment: 4 pages, 4 EPS figures included, REVTe
Fractal Dimensions of Confined Clusters in Two-Dimensional Directed Percolation
The fractal structure of directed percolation clusters, grown at the
percolation threshold inside parabolic-like systems, is studied in two
dimensions via Monte Carlo simulations. With a free surface at y=\pm Cx^k and a
dynamical exponent z, the surface shape is a relevant perturbation when k<1/z
and the fractal dimensions of the anisotropic clusters vary continuously with
k. Analytic expressions for these variations are obtained using a blob picture
approach.Comment: 6 pages, Plain TeX file, epsf, 3 postscript-figure
A model of semimetallic behavior in strongly correlated electron systems
Metals with values of the resistivity and the Hall coefficient much larger
than typical ones, e.g., of sodium, are called semimetals. We suggest a model
for semimetals which takes into account the strong Coulomb repulsion of the
charge carriers, especially important in transition-metal and rare-earth
compounds. For that purpose we extend the Hubbard model by coupling one
additional orbital per site via hybridization to the Hubbard orbitals. We
calculate the spectral function, resistivity and Hall coefficient of the model
using dynamical mean-field theory. Starting from the Mott-insulating state, we
find a transition to a metal with increasing hybridization strength
(``self-doping''). In the metallic regime near the transition line to the
insulator the model shows semimetallic behavior. We compare the calculated
temperature dependence of the resistivity and the Hall coefficient with the one
found experimentally for . The comparison demonstrates that the
anomalies in the transport properties of possibly can be
assigned to Coulomb interaction effects of the charge carriers not captured by
standard band structure calculations.Comment: 9 pages RevTeX with 7 ps figures, accepted by PR
Gap to Transition Temperature Ratio in Density Wave Ordering: a Dynamical Mean Field Study
We use the dynamical mean-field method to determine the origin of the large
ratio of the zero temperature gap to the transition temperature observed in
most charge density wave materials. The method is useful because it allows an
exact treatment of thermal fluctuations. We establish the relation of the
dynamical mean-field results to conventional diagrammatics and thereby
determine that in the physically relevant regime the origin of the large ratio
is a strong inelastic scattering.Comment: 4 figure
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