88 research outputs found
Difference of optical conductivity between one- and two-dimensional doped nickelates
We study the optical conductivity in doped nickelates, and find the dramatic
difference of the spectrum in the gap (\alt4 eV) between one- (1D)
and two-dimensional (2D) nickelates. The difference is shown to be caused by
the dependence of hopping integral on dimensionality. The theoretical results
explain consistently the experimental data in 1D and
2D nickelates, YCaBaNiO and LaSrNiO,
respectively. The relation between the spectrum in the X-ray aborption
experiments and the optical conductivity in LaSrNiO is
discussed.Comment: RevTeX, 4 pages, 4 figure
Theory of the optical conductivity of (TMTSF)PF in the mid-infrared range
We propose an explanation of the mid-infrared peak observed in the optical
conductivity of the Bechgaard salt (TMTSF)PF in terms of electronic
excitations. It is based on a numerical calculation of the conductivity of the
quarter-filled, dimerized Hubbard model. The main result is that, even for
intermediate values of for which the charge gap is known to be very
small, the first peak, and at the same time the main structure, of the optical
conductivity is at an energy of the order of the dimerization gap, like in the
infinite case. This surprising effect is a consequence of the optical
selection rules.Comment: 10 pages, 9 uuencoded figure
Singular Structure and Enhanced Friedel Oscillations in the Two-Dimensional Electron Gas
We calculate the leading order corrections (in ) to the static
polarization , with dynamically screened interactions, for the
two-dimensional electron gas. The corresponding diagrams all exhibit singular
logarithmic behavior in their derivatives at and provide significant
enhancement to the proper polarization particularly at low densities. At a
density of , the contribution from the leading order {\em fluctuational}
diagrams exceeds both the zeroth order (Lindhard) response and the self-energy
and exchange contributions. We comment on the importance of these diagrams in
two-dimensions and make comparisons to an equivalent three-dimensional electron
gas; we also consider the impact these finding have on computed
to all orders in perturbation theory
Double Exchange Model for Magnetic Hexaborides
A microscopic theory for rare-earth ferromagnetic hexaborides, such as
Eu(1-x)Ca(x)B6, is proposed on the basis of the double-exchange Hamiltonian. In
these systems, the reduced carrier concentrations place the Fermi level near
the mobility edge, introduced in the spectral density by the disordered spin
background. We show that the transport properties such as Hall effect,
magnetoresitance, frequency dependent conductivity, and DC resistivity can be
quantitatively described within the model. We also make specific predictions
for the behavior of the Curie temperature, Tc, as a function of the plasma
frequency, omega_p.Comment: 4 pages, 3 figure
Role of Collective Mode for Optical Conductivity and Reflectivity in Quarter-Filled Spin-Density-Wave State
Taking account of a collective mode relevant to charge fluctuation, the
optical conductivity of spin-density-wave state has been examined for an
extended Hubbard model with one-dimensional quarter-filled band. We find that,
within the random phase approximation, the conductivity exhibits several peaks
at the frequency corresponding to the excitation energy of the commensurate
collective mode. When charge ordering appears with increasing inter-site
repulsive interactions, the main peak with the lowest frequency is reduced and
the effective mass of the mode is enhanced indicating the suppression of the
effect of the collective mode by charge ordering. It is also shown that the
reflectivity becomes large in a wide range of frequency due to the huge
dielectric constant induced by the collective mode.Comment: 11 pages, 16 figure
Deducing correlation parameters from optical conductivity in the Bechgaard salts
Numerical calculations of the kinetic energy of various extensions of the
one-dimensional Hubbard model including dimerization and repulsion between
nearest neighbours are reported. Using the sum rule that relates the kinetic
energy to the integral of the optical conductivity, one can determine which
parameters are consistent with the reduction of the infrared oscillator
strength that has been observed in the Bechgaard salts. This leads to improved
estimates of the correlation parameters for both the TMTSF and TMTTF series.Comment: 12 pages, latex, figures available from the author
Quasiparticle undressing in a dynamic Hubbard model: exact diagonalization study
Dynamic Hubbard models have been proposed as extensions of the conventional
Hubbard model to describe the orbital relaxation that occurs upon double
occupancy of an atomic orbital. These models give rise to pairing of holes and
superconductivity in certain parameter ranges. Here we explore the changes in
carrier effective mass and quasiparticle weight and in one- and two-particle
spectral functions that occur in a dynamic Hubbard model upon pairing, by exact
diagonalization of small systems. It is found that pairing is associated with
lowering of effective mass and increase of quasiparticle weight, manifested in
transfer of spectral weight from high to low frequencies in one- and
two-particle spectral functions. This 'undressing' phenomenology resembles
observations in transport, photoemission and optical experiments in high T_c
cuprates. This behavior is contrasted with that of a conventional electron-hole
symmetric Holstein-like model with attractive on-site interaction, where
pairing is associated with 'dressing' instead of 'undressing'
Scaling theory of the Mott-Hubbard metal-insulator transition in one dimension
We use the Bethe ansatz equations to calculate the charge stiffness of the one-dimensional
repulsive-interaction Hubbard model for electron densities close to the Mott
insulating value of one electron per site (), where is the ground
state energy, is the circumference of the system (assumed to have periodic
boundary conditions), and is the magnetic flux
enclosed. We obtain an exact result for the asymptotic form of
as at , which defines and yields an analytic expression for
the correlation length in the Mott insulating phase of the model as a
function of the on-site repulsion . In the vicinity of the zero temperature
critical point U=0, , we show that the charge stiffness has the
hyperscaling form , where and is a universal scaling function which we calculate. The
physical significance of in the metallic phase of the model is that it
defines the characteristic size of the charge-carrying solitons, or {\em
holons}. We construct an explicit mapping for arbitrary and of the holons onto weakly interacting spinless fermions, and use this
mapping to obtain an asymptotically exact expression for the low temperature
thermopower near the metal-insulator transition, which is a generalization to
arbitrary of a result previously obtained using a weak- coupling
approximation, and implies hole-like transport for .Comment: 34 pages, REVTEX (5 figures by request
Electronic dynamic Hubbard model: exact diagonalization study
A model to describe electronic correlations in energy bands is considered.
The model is a generalization of the conventional Hubbard model that allows for
the fact that the wavefunction for two electrons occupying the same Wannier
orbital is different from the product of single electron wavefunctions. We
diagonalize the Hamiltonian exactly on a four-site cluster and study its
properties as function of band filling. The quasiparticle weight is found to
decrease and the quasiparticle effective mass to increase as the electronic
band filling increases, and spectral weight in one- and two-particle spectral
functions is transfered from low to high frequencies as the band filling
increases. Quasiparticles at the Fermi energy are found to be more 'dressed'
when the Fermi level is in the upper half of the band (hole carriers) than when
it is in the lower half of the band (electron carriers). The effective
interaction between carriers is found to be strongly dependent on band filling
becoming less repulsive as the band filling increases, and attractive near the
top of the band in certain parameter ranges. The effective interaction is most
attractive when the single hole carriers are most heavily dressed, and in the
parameter regime where the effective interaction is attractive, hole carriers
are found to 'undress', hence become more like electrons, when they pair. It is
proposed that these are generic properties of electronic energy bands in solids
that reflect a fundamental electron-hole asymmetry of condensed matter. The
relation of these results to the understanding of superconductivity in solids
is discussed.Comment: Small changes following referee's comment
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