271 research outputs found
A Variational Approach to Nonlocal Exciton-Phonon Coupling
In this paper we apply variational energy band theory to a form of the
Holstein Hamiltonian in which the influence of lattice vibrations (optical
phonons) on both local site energies (local coupling) and transfers of
electronic excitations between neighboring sites (nonlocal coupling) is taken
into account. A flexible spanning set of orthonormal eigenfunctions of the
joint exciton-phonon crystal momentum is used to arrive at a variational
estimate (bound) of the ground state energy for every value of the joint
crystal momentum, yielding a variational estimate of the lowest polaron energy
band across the entire Brillouin zone, as well as the complete set of polaron
Bloch functions associated with this band. The variation is implemented
numerically, avoiding restrictive assumptions that have limited the scope of
previous assaults on the same and similar problems. Polaron energy bands and
the structure of the associated Bloch states are studied at general points in
the three-dimensional parameter space of the model Hamiltonian (electronic
tunneling, local coupling, nonlocal coupling), though our principal emphasis
lay in under-studied area of nonlocal coupling and its interplay with
electronic tunneling; a phase diagram summarizing the latter is presented. The
common notion of a "self-trapping transition" is addressed and generalized.Comment: 33 pages, 11 figure
Comment on `Dynamical properties of small polarons'
We show that the conclusion on the breakdown of the standard small polaron
theory made recently by E.V. deMello and J. Ranninger (Phys. Rev. B 55, 14872
(1997)) is a result of an incorrect interpretation of the electronic and
vibronic energy levels of the two-site Holstein model. The small polaron
theory, when properly applied, agrees well with the numerical results of these
authors. Also we show that their attempt to connect the properties of the
calculated correlation functions with the features of the intersite electron
hopping is unsuccessful.Comment: To appear in Phys. Rev.
Disorder-driven superconductor-normal metal phase transition in quasi-one-dimensional organic conductors
Effects of non-magnetic disorder on the critical temperature T_c and on
diamagnetism of quasi-one-dimensional superconductors are reported. The energy
of Josephson-coupling between wires is considered to be random, which is
typical for dirty organic superconductors. We show that this randomness
destroys phase coherence between wires and that T_c vanishes discontinuously at
a critical disorder-strength. The parallel and transverse components of the
penetration-depth are evaluated. They diverge at different critical
temperatures T_c^{(1)} and T_c, which correspond to pair-breaking and
phase-coherence breaking respectively. The interplay between disorder and
quantum phase fluctuations is shown to result in quantum critical behavior at
T=0, which manifests itself as a superconducting-normal metal phase transition
of first-order at a critical disorder strength.Comment: 12 pages, 3 figure
Multiple Current States of Two Phase-Coupled Superconducting Rings
The states of two phase-coupled superconducting rings have been investigated.
Multiple current states have been revealed in the dependence of the critical
current on the magnetic field. The performed calculations of the critical
currents and energy states in a magnetic field have made it possible to
interpret the experiment as the measurement of energy states into which the
system comes with different probabilities because of the equilibrium and
non-equilibrium noises upon the transition from the resistive state to the
superconducting state during the measurement of the critical currentComment: 5 pages, 5 figure
Luttinger-liquid-like transport in long InSb nanowires
Long nanowires of degenerate semiconductor InSb in asbestos matrix (wire
diameter is around 50 \AA, length 0.1 - 1 mm) were prepared. Electrical
conduction of these nanowires is studied over a temperature range 1.5 - 350 K.
It is found that a zero-field electrical conduction is a power function of the
temperature with the typical exponent .
Current-voltage characteristics of such nanowires are found to be nonlinear and
at sufficiently low temperatures follows the power law . It
is shown that the electrical conduction of these nanowires cannot be accounted
for in terms of ordinary single-electron theories and exhibits features
expected for impure Luttinger liquid. For a simple approximation of impure LL
as a pure one broken into drops by weak links, the estimated weak-link density
is around per cm.Comment: 5 pages, 2 figure
Unconventional magnetoresistance in long InSb nanowires
Magnetoresistance in long correlated nanowires of degenerate semiconductor
InSb in asbestos matrix (wire diameter of around 5 nm, length 0.1 - 1 mm) is
studied over temperature range 2.3 - 300 K. At zero magnetic field the electric
conduction and the current-voltage characteristics of such wires obey the
power laws , , expected for
one-dimensional electron systems. The effect of magnetic field corresponds to a
20% growth of the exponents , at H=10 T. The observed
magnetoresistance is caused by the magnetic-field-induced breaking of the
spin-charge separation and represents a novel mechanism of magnetoresistance.Comment: To be published in JETP Letters, vol. 77 (2003
Polaron and bipolaron dispersion curves in one dimension for intermediate coupling
Bipolaron energies are calculated as a function of wave vector by a
variational method of Gurari appropriate for weak or intermediate coupling
strengths, for a model with electron-phonon interactions independent of phonon
wave vectors and a short-ranged Coulomb repulsion. It is assumed that the bare
electrons have a constant effective mass. A two-parameter trial function is
taken for the relative motion of the two electrons in the bipolaron. Energies
of bipolarons are compared with those of two single polarons as a function of
wave vector for various parameter values. Results for effective masses at the
zone center are also obtained. Comparison is made with data of other authors
for bipolarons in the Hubbard-Holstein model, which differs mainly from the
present model in that it has a tight-binding band structure for the bare
electrons.Comment: 11 pages including six figures. Physical Review B, to be publishe
pH and rate of ‘dark’ events in toad retinal rods : test of a hypothesis on the molecular origin of photoreceptor noise
Thermal activation of the visual pigment constitutes a fundamental constraint on visual sensitivity.
Its electrical correlate in the membrane current of dark-adapted rods are randomly occurring
discrete ‘dark events’ indistinguishable from responses to single photons. It has been proposed that
thermal activation occurs in a small subpopulation of rhodopsin molecules where the Schiff base
linking the chromophore to the protein part is unprotonated. On this hypothesis, rates of thermal
activation should increase strongly with rising pH. The hypothesis has been tested by measuring the
effect of pH changes on the frequency of discrete dark events in red rods of the common toad Bufo
bufo. Dark noise was recorded from isolated rods using the suction pipette technique. Changes in
cytoplasmic pH upon manipulations of extracellular pH were quantified by measuring, using
fast single-cell microspectrophotometry, the pH-dependent metarhodopsin I–metarhodopsin II
equilibrium and subsequent metarhodopsin III formation. These measurements show that, in the
conditions of the electrophysiological experiments, changing perfusion pH from 6.5 to 9.3 resulted
in a cytoplasmic pH shift from 7.6 to 8.5 that was readily sensed by the rhodopsin. This shift, which
implies an 8-fold decrease in cytoplasmic [H+], did not increase the rate of dark events. The results
contradict the hypothesis that thermal pigment activation depends on prior deprotonation of the
Schiff base
Comparison of perturbative expansions using different phonon bases for two-site Holstein model
The two-site single-polaron problem is studied within the perturbative
expansions using different standard phonon basis obtained through the Lang
Firsov (LF), modified LF (MLF) and modified LF transformation with squeezed
phonon states (MLFS). The role of these convergent expansions using the above
prescriptions in lowering the energy and in determining the correlation
functions are compared for different values of coupling strength. The
single-electron energy, oscillator wave functions and correlation functions are
calculated for the same system. The applicability of different phonon basis in
different regimes of the coupling strength as well as in different regimes of
hopping are also discussed.Comment: 24 pages (RevTEX), 12 postscript figures, final version accepted in
PRB(2000) Jornal Ref: Phys. Rev. B, 61, 4592-4602 (2000
Positive Magneto-Resistance in Quasi-1D Conductors
We present here a simple qualitative model that interpolates between the high
and low temperature properties of quasi-1D conductors. At high temperatures we
argue that transport is governed by inelastic scattering whereas at low
temperatures the conductance decays exponentially with the electron dephasing
length. The crossover between these regimes occurs at the temperature at which
the elastic and inelastic scattering times become equal. This model is shown to
be in quantitative agreement with the organic conductor .
Within this model, we also show that on the insulating side, the positive
magnetoresistance of the form observed in and
other quasi-1D conductors can be explained by the role spin-flip scattering
plays in the electron dephasing rate.Comment: 4 pages, Latex, no figure
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