256 research outputs found
Iterative Perturbation Theory for Strongly Correlated Electron Systems with Orbital Degeneracy
A new scheme of the iterative perturbation theory is proposed for the
strongly correlated electron systems with orbital degeneracy. The method is
based on the modified self-energy of Yeyati, et al. which interpolates between
the weak and the strong correlation limits, but a much simpler scheme is
proposed which is useful in the case of the strong correlation with orbital
degeneracy. It will be also useful in the study of the electronic structures
combined with the band calculations.Comment: 6 pages, 3 Postscript figures, to appear in J. Phys. Cond. Matte
Theory for Magnetic Anisotropy of Field-Induced Insulator-to-Metal Transition in Cubic Kondo Insulator YbB_{12}
Magnetization and energy gap of Kondo insulator YbB_{12} are calculated
theoretically based on the previously proposed tight-binding model composed of
Yb 5d and 4f orbitals. It is found that magnetization
curves are almost isotropic, naturally expected from the cubic symmetry, but
that the gap-closing field has an anisotropy: the gap closes faster for the
field in (100) direction than in (110) and (111) directions, in accord with the
experiments. This is qualitatively understood by considering the maximal
eigenvalues of the total angular momentum operators projected on each direction
of the magnetic field. But the numerical calculation based on the band model
yields better agreement with the experiment.Comment: 4 pages, 4 figures, to appear in J. Phys. Soc. Jp
Calculation of Optical Conductivity of YbB using Realistic Tight-Binding Model
Based on the previously reported tight-binding model fitted to the LDA+U band
calculation, optical conductivity of the prototypical Kondo insulator
YbB is calculated theoretically. Many-body effects are taken into
account by the self-consistent second order perturbation theory. The gross
shape of the optical conductivity observed in experiments are well described by
the present calculation, including their temperature-dependences.Comment: 6 pages, 7 figures, use jpsj2.cls, to appear in J. Phys. Soc. Jpn.
Vol.73, No.10 (2004
Optical conductivity of filled skutterudites
A simple tight-binding model is constructed for the description of the
electronic structure of some Ce-based filled skutterudite compounds showing an
energy gap or pseudogap behavior. Assuming band-diagonal electron interactions
on this tight-binding model, the optical conductivity spectrum is calculated by
applying the second-order self-consistent perturbation theory to treat the
electron correlation. The correlation effect is found to be of great importance
on the description of the temperature dependence of the optical conductivity.
The rapid disappearance of an optical gap with increasing temperature is
obtained as observed in the optical experiment for Ce-based filled-skutterudite
compounds.Comment: 6 pages, 7 figures, use jpsj2.cls, to appear in J. Phys. Soc. Jpn.
Vol.73, No.10 (2004
Calculation of Optical Conductivity, Resistivity and Thermopower of Filled Skutterudite CeRuSb based on a Realistic Tight-binding Model with Strong Correlation
The filled-skutterudite compound CeRuSb shows a pseudo-gap
structure in the optical conductivity spectra similar to the Kondo insulators,
but metallic behavior below 80 K. The resistivity shows a large peak at 80 K,
and the Seebeck coefficient is positive and also shows a large peak at nearly
the same temperature. In order to explain all these features, a simplified
tight-binding model, which captures the essential features of the band
calculation, is proposed. Using this model and introducing the correlation
effect within the framework of the dynamical mean field approximation and the
iterative perturbation theory, the temperature dependences of the optical
conductivity, resistivity and the Seebeck coefficient are calculated, which can
explain the experiments.Comment: 4 pages, 6 figure
Absence of Hybridization Gap in Heavy Electron Systems and Analysis of YbAl3 in terms of Nearly Free Electron Conduction Band
In the analysis of the heavy electron systems, theoretical models with c-f
hybridization gap are often used. We point out that such a gap does not exist
and the simple picture with the hybridization gap is misleading in the metallic
systems, and present a correct picture by explicitly constructing an effective
band model of YbAl_3. Hamiltonian consists of a nearly free electron model for
conduction bands which hybridize with localized f-electrons, and includes only
a few parameters. Density of states, Sommerfeld coefficient, f-electron number
and optical conductivity are calculated and compared with the band calculations
and the experiments.Comment: 9 pages, 9 figures, submitted to J. Phys. Soc. Jp
Kondo Effect in Fermi Systems with a Gap: A Renormalization Group Study
We present the results of a Wilson Renormalization Group study of the
single-impurity Kondo and Anderson models in a system with a gap in the
conduction electron spectrum. The behavior of the impurity susceptibility and
the zero-frequency response function, are discussed in the
cases with and without particle-hole symmetry. In addition, for the asymmetric
Anderson model the correlation functions, , are computed.Comment: 10 pages, 10 figure
Thermal and Dynamical Properties of the Two-band Hubbard Model Compared with FeSi
We study the two-band Hubbard model introduced by Fu and Doniach as a model
for FeSi which is suggested to be a Kondo insulator. Using the self-consistent
second-order perturbation theory combined with the local approximation which
becomes exact in the limit of infinite dimensions, we calculate the specific
heat, the spin susceptibility and the dynamical conductivity and point out that
the reduction of the energy gap due to correlation is not significant in
contrast to the previous calculation. It is also demonstrated that the gap at
low temperatures in the optical conductivity is filled up at a rather low
temperature than the gap size, which is consistent with the experiment.Comment: 6 pages, LaTeX, 7 PS figures included, uses RevTe
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