7 research outputs found
Polaron and Bipolaron Defects in a Charge Density Wave: a Model for Lightly Doped BaBiO3
BaBiO3 is a prototype ``charge ordering system'' forming interpenetrating
sublattices with nominal valence Bi(3+) and Bi(5+). It can also be regarded as
a three-dimensional version of a Peierls insulator, the insulating gap being a
consequence of an ordered distortion of oxygen atoms. When holes are added to
BaBiO3 by doping, it remains insulating until a very large hole concentration
is reached, at which point it becomes superconducting. The mechanism for
insulating behavior of more lightly-doped samples is formation of small
polarons or bipolarons. These are self-organized point defects in the Peierls
order parameter, which trap carriers in bound states inside the Peierls gap. We
calculate properties of the polarons and bipolarons using the Rice-Sneddon
model. Bipolarons are the stable defect; the missing pair of electrons come
from an empty midgap state built from the lower Peierls band. Each bipolaron
distortion also pulls down six localized states below the bottom of the
unoccupied upper Peierls band. The activation energy for bipolaron hopping is
estimated.Comment: 9 pages with 8 embedded figures. See also cond-mat/0108089, a paper
of 5 pages on the related topic of self-trapped excitons in BaBiO
Phonon `notches' in a-b -plane optical conductivity of high-Tc superconductors
It is shown that a correlation between the positions of the -axis
longitudinal optic () phonons and ``notch''-like structures in the
- plane conductivity of high- superconductors results from
phonon-mediated interaction between electrons in different layers. It is found
that the relative size of the notches depends on
, where ,
and are the effective coupling strength, the frequency and the
width of the optical phonon which is responsible for the notch. Even for
the effect can be large if the phonon is very sharp.Comment: 5 pages, REVTeX, 4 uuencoded figure
Topological (Sliced) Doping of a 3D Peierls System: Predicted Structure of Doped BaBiO3
At hole concentrations below x=0.4, Ba_(1-x)K_xBiO_3 is non-metallic. At x=0,
pure BaBiO3 is a Peierls insulator. Very dilute holes create bipolaronic point
defects in the Peierls order parameter. Here we find that the Rice-Sneddon
version of Peierls theory predicts that more concentrated holes should form
stacking faults (two-dimensional topological defects, called slices) in the
Peierls order parameter. However, the long-range Coulomb interaction, left out
of the Rice-Sneddon model, destabilizes slices in favor of point bipolarons at
low concentrations, leaving a window near 30% doping where the sliced state is
marginally stable.Comment: 6 pages with 5 embedded postscript figure
Spectral properties of the t-J model in the presence of hole-phonon interaction
We examine the effects of electron-phonon interaction on the dynamics of the
charge carriers doped in two-dimensional (2D) Heisenberg antiferromagnet. The
- model Hamiltonian with a Fr\"ohlich term which couples the holes to a
dispersionless (optical) phonon mode is considered for low doping
concentration. The evolution of the spectral density function, the density of
states, and the momentum distribution function of the holes with an increase of
the hole-phonon coupling constant is studied numerically. As the coupling
to a phonon mode increases the quasiparticle spectral weight decreases and a
``phonon satellite'' feature close to the quasi-particle peak becomes more
pronounced. Furthermore, strong electron-phonon coupling smears the
multi-magnon resonances (``string states'') in the incoherent part of the
spectral function. The jump in the momentum distribution function at the Fermi
surface is reduced without changing the hole pocket volume, thereby providing a
numerical verification of Luttinger theorem for this strongly interacting
system. The vertex corrections due to electron- phonon interaction are
negligible in spite of the fact that the ratio of the phonon frequency to the
effective bandwidth is not small.Comment: REVTeX, 20 pages, 9 figures, to be published in Phys. Rev. B (Nov. 1,
1996