377 research outputs found
Self-Trapped Exciton Defects in a Charge Density Wave: Electronic Excitations of BaBiO3
In the previous paper, it was shown that holes doped into BaBiO3 self-trap as
small polarons and bipolarons. These point defects are energetically favorable
partly because they undo locally the strain in the charge-density-wave (Peierls
insulator) ground state. In this paper the neutral excitations of the same
model are discussed. The lowest electronic excitation is predicted to be a
self-trapped exciton, consisting of an electron and a hole located on adjacent
Bi atoms. This excitation has been seen experimentally (but not identified as
such) via the Urbach tail in optical absorption, and the multi-phonon spectrum
of the ``breathing mode'' seen in Raman scattering. These two phenomena occur
because of the Franck-Condon effect associated with oxygen displacement in the
excited state.Comment: 5 pages with 7 embedded figures. See also cond-mat/0108089 on
polarons and bipolarons in BaBiO3 contains background informatio
Variable-range hopping in quasi-one-dimensional electron crystals
We study the effect of impurities on the ground state and the low-temperature
dc transport in a 1D chain and quasi-1D systems of many parallel chains. We
assume that strong interactions impose a short-range periodicicity of the
electron positions. The long-range order of such an electron crystal (or
equivalently, a charge-density wave) is destroyed by impurities. The 3D
array of chains behaves differently at large and at small impurity
concentrations . At large , impurities divide the chains into metallic
rods. The low-temperature conductivity is due to the variable-range hopping of
electrons between the rods. It obeys the Efros-Shklovskii (ES) law and
increases exponentially as decreases. When is small, the metallic-rod
picture of the ground state survives only in the form of rare clusters of
atypically short rods. They are the source of low-energy charge excitations. In
the bulk the charge excitations are gapped and the electron crystal is pinned
collectively. A strongly anisotropic screening of the Coulomb potential
produces an unconventional linear in energy Coulomb gap and a new law of the
variable-range hopping . remains
constant over a finite range of impurity concentrations. At smaller the
2/5-law is replaced by the Mott law, where the conductivity gets suppressed as
goes down. Thus, the overall dependence of on is nonmonotonic.
In 1D, the granular-rod picture and the ES apply at all . The conductivity
decreases exponentially with . Our theory provides a qualitative explanation
for the transport in organic charge-density wave compounds.Comment: 20 pages, 7 figures. (v1) The abstract is abridged to 24 lines. For
the full abstract, see the manuscript (v2) several changes in presentation
per referee's comments. No change in result
Ionization degree of the electron-hole plasma in semiconductor quantum wells
The degree of ionization of a nondegenerate two-dimensional electron-hole
plasma is calculated using the modified law of mass action, which takes into
account all bound and unbound states in a screened Coulomb potential.
Application of the variable phase method to this potential allows us to treat
scattering and bound states on the same footing. Inclusion of the scattering
states leads to a strong deviation from the standard law of mass action. A
qualitative difference between mid- and wide-gap semiconductors is
demonstrated. For wide-gap semiconductors at room temperature, when the bare
exciton binding energy is of the order of T, the equilibrium consists of an
almost equal mixture of correlated electron-hole pairs and uncorrelated free
carriers.Comment: 22 pages, 6 figure
Ground-state phase diagram of the one-dimensional half-filled extended Hubbard model
We revisit the ground-state phase diagram of the one-dimensional half-filled
extended Hubbard model with on-site (U) and nearest-neighbor (V) repulsive
interactions. In the first half of the paper, using the weak-coupling
renormalization-group approach (g-ology) including second-order corrections to
the coupling constants, we show that bond-charge-density-wave (BCDW) phase
exists for U \approx 2V in between charge-density-wave (CDW) and
spin-density-wave (SDW) phases. We find that the umklapp scattering of
parallel-spin electrons disfavors the BCDW state and leads to a bicritical
point where the CDW-BCDW and SDW-BCDW continuous-transition lines merge into
the CDW-SDW first-order transition line. In the second half of the paper, we
investigate the phase diagram of the extended Hubbard model with either
additional staggered site potential \Delta or bond alternation \delta. Although
the alternating site potential \Delta strongly favors the CDW state (that is, a
band insulator), the BCDW state is not destroyed completely and occupies a
finite region in the phase diagram. Our result is a natural generalization of
the work by Fabrizio, Gogolin, and Nersesyan [Phys. Rev. Lett. 83, 2014
(1999)], who predicted the existence of a spontaneously dimerized insulating
state between a band insulator and a Mott insulator in the phase diagram of the
ionic Hubbard model. The bond alternation \delta destroys the SDW state and
changes it into the BCDW state (or Peierls insulating state). As a result the
phase diagram of the model with \delta contains only a single critical line
separating the Peierls insulator phase and the CDW phase. The addition of
\Delta or \delta changes the universality class of the CDW-BCDW transition from
the Gaussian transition into the Ising transition.Comment: 24 pages, 20 figures, published versio
Field dependence of the vortex structure in chiral p-wave superconductors
To investigate the different vortex structure between two chiral pairing p_x
+(-) i p_y, we calculate the pair potential, the internal field, the local
density of states, and free energy in the vortex lattice state based on the
quasiclassical Eilenberger theory, and analyze the magnetic field dependence.
The induced opposite chiral component of the pair potential plays an important
role in the vortex structure. It also produces H^{1/2}-behavior of the
zero-energy density of states at higher field. These results are helpful when
we understand the vortex states in Sr2RuO4.Comment: 11 pages, 10 figures, to be published in Phys. Rev.
Edge states and determination of pairing symmetry in superconducting Sr2RuO4
We calculate the energy dispersion of the surface Andreev states and their
contribution to tunneling conductance for the order parameters with horizontal
and vertical lines of nodes proposed for superconducting Sr2RuO4. For vertical
lines, we find double peaks in tunneling spectra reflecting the van Hove
singularities in the density of surface states originating from the turning
points in their energy dispersion. For horizontal lines, we find a single
cusp-like peak at zero bias, which agrees very well with the experimental data
on tunneling in Sr2RuO4.Comment: 6 pages, 6 figures. V.2: comparison with experiment added and
discussion of horizontal nodes expanded. v.3: significant expansion: 1 figure
and 2 pages added. v.4: acknowledgements added. Additional viewgraphs with
experimental and theoretical curves superimposed are available at
http://www2.physics.umd.edu/~yakovenk/talks/Sr2RuO4
Low temperature electronic properties of Sr_2RuO_4 II: Superconductivity
The body centered tetragonal structure of Sr_2RuO_4 gives rise to umklapp
scattering enhanced inter-plane pair correlations in the d_{yz} and d_{zx}
orbitals. Based on symmetry arguments, Hund's rule coupling, and a bosonized
description of the in-plane electron correlations the superconducting order
parameter is found to be a orbital-singlet spin-triplet with two spatial
components. The spatial anisotropy is 7%. The different components of the order
parameter give rise to two-dimensional gapless fluctuations. The phase
transition is of third order. The temperature dependence of the pair density,
specific heat, NQR, Knight shift, and susceptibility are in agreement with
experimental results.Comment: 20 pages REVTEX, 3 figure
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