152 research outputs found
Excitonic Bound State in the Extended Anderson Model with c-f Coulomb Interaction
The Anderson model with the Coulomb interaction between the local and
conduction electrons is studied in the semiconducting phase. Based on a
perturbation theory from the atomic limit, leading contributions for the c-f
Coulomb interaction are incorporated as a vertex correction to hybridization.
An analytical solution shows that the effective attraction in the intermediate
states leads to a bound state localized at the local electron site.
Self-consistent equations are constructed as an extension of the non-crossing
approximation (NCA) to include the vertex part yielding the bound state. A
numerical calculation demonstrates the excitonic bound state inside the
semiconducting gap for single-particle excitations, and a discontinuity at the
gap edge for magnetic excitations.Comment: 15 pages, 20 figures, submitted to J. Phys. Soc. Jp
Insulator-to-metal transition in Kondo insulators under strong magnetic field
Magnetization curve and changes of the single-particle excitation spectra by
magnetic field are calculated for the periodic Anderson model at half-filling
in infinite spatial dimension by using the exact diagonalization method. It is
found that the field-induced insulator-to-metal transition occurs at a critical
field , which is of the order of the single ion Kondo temperature. The
transition is of first order, but could be of second order in the infinite
system size limit. These results are compared with the experiments on the Kondo
insulator YbB.Comment: 11 pages, REVTEX, no figures; 7 figures available on request; To
appear in Phys. Rev. B, Mar.15, 199
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
Formation Mechanism of Hybridization Gap in Kondo Insulators based on a Realistic Band Model and Application to YbB
A new LDA+U band calculation is performed on the Kondo insulator material
YbB and an energy gap of about 0.001Ryd is obtained. Based on this, a
simple tight-binding model with 5d and 4f orbitals on Yb
atoms and the nearest neighbor -bonds between them is constructed with
a good agreement to the above the LDA+U calculation near the gap. The density
of states is also calculated and the shape is found to be very asymmetric with
respect to the gap. A formation mechanism of the gap is clarified for the first
time in a realistic situation with the orbital degeneracies in both conduction
bands and the f states. This model can be a useful starting point for
incorporating the strong correlation effect, and for understanding all the
thermal, thermoelectric, transport and magnetic properties of YbB.Comment: 15 pages, 15 figures, to appear in J. Phys. Soc. Jpn. Vol. 72 No. 5
(2003
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
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
Correlation Effects on Optical Conductivity of FeSi
Effects of electron correlation in FeSi are studied in terms of the two-band
Hubbard model with the density of states obtained from the band calculation.
Using the self-consistent second-order perturbation theory combined with the
local approximation, the correlation effects are investigated on the density of
states and the optical conductivity spectrum, which are found to reproduce the
experiments done by Damascelli et al. semiquantitatively. It is also found that
the peak at the gap edge shifts to lower energy region by correlation effects,
as is seen in the experiments.Comment: 4 pages, 3 figure
Unusual Field-Insensitive Phase Transition and Kondo Behavior in SmTiAl
Magnetization, electrical resistivity and specific heat measurements were
performed on high-quality single crystalline SmTiAl (residual
resistivity ratio 40) grown by Al self-flux method. A Kondo-like dependence in the resistivity is observed below 50 K. We discovered a
field-insensitive phase transition at = 6.5 K and a field-insensitive
heavy fermion behavior with the electronic specific heat coefficient =
150 mJ/(K mol). Specific heat analysis reveals that the ground state is a
quartet state and the Sm magnetic dipole moment
( at 0) orders below in spite of the
field-insensitive behavior. Possible reasons for the field insensitiveness will
be discussed.Comment: 4 pages, 3 figures, to be published in J. Phys. Soc. Jpn. 80 (2011
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