493 research outputs found
Momentum average approximation for models with electron-phonon coupling dependent on the phonon momentum
We generalize the momentum average (MA) approximation to study the properties
of models with momentum-dependent electron-phonon coupling. As in the case of
the application of the original MA to the Holstein model, the results are
analytical, numerically trivial to evaluate, exact for both zero bandwidth and
for zero electron-phonon coupling, and are accurate everywhere in parameter
space. Comparison with available numerical data confirms this accuracy. We then
show that further improvements can be obtained based on variational
considerations, using the one-dimensional breathing-mode Hamiltonian as a
specific example. For example, by using this variational MA, we obtain ground
state energies within at most 0.3% error of the numerical data.Comment: 15 pages, 10 figure
Weak and strong coupling limits of the two-dimensional Fr\"ohlich polaron with spin-orbit Rashba interaction
The continuous progress in fabricating low-dimensional systems with large
spin-orbit couplings has reached a point in which nowadays materials may
display spin-orbit splitting energies ranging from a few to hundreds of meV.
This situation calls for a better understanding of the interplay between the
spin-orbit coupling and other interactions ubiquitously present in solids, in
particular when the spin-orbit splitting is comparable in magnitude with
characteristic energy scales such as the Fermi energy and the phonon frequency.
In this article, the two-dimensional Fr\"ohlich electron-phonon problem is
reformulated by introducing the coupling to a spin-orbit Rashba potential,
allowing for a description of the spin-orbit effects on the electron-phonon
interaction. The ground state of the resulting Fr\"ohlich-Rashba polaron is
studied in the weak and strong coupling limits of the electron-phonon
interaction for arbitrary values of the spin-orbit splitting. The weak coupling
case is studied within the Rayleigh-Schr\"odinger perturbation theory, while
the strong-coupling electron-phonon regime is investigated by means of
variational polaron wave functions in the adiabatic limit. It is found that,
for both weak and strong coupling polarons, the ground state energy is
systematically lowered by the spin-orbit interaction, indicating that the
polaronic character is strengthened by the Rashba coupling. It is also shown
that, consistently with the lowering of the ground state, the polaron effective
mass is enhanced compared to the zero spin-orbit limit. Finally, it is argued
that the crossover between weakly and strongly coupled polarons can be shifted
by the spin-orbit interaction.Comment: 11 pages, 5 figure
Sub-nanosecond delay of light in (Cd,Zn)Te crystal
We study excitonic polariton relaxation and propagation in bulk CdZnTe using
time- resolved photoluminescence and time-of-flight techniques. Propagation of
picosecond optical pulses through 0.745 mm thick crystal results in time delays
up to 350 ps, depending on the photon energy. Optical pulses with 150 fs
duration become strongly stretched. The spectral dependence of group velocity
is consistent with the dispersion of the lower excitonic polariton branch. The
lifetimes of excitonic polariton in the upper and lower branches are 1.5 and 3
ns, respectively.Comment: 5 pages, 4 figure
Correlation of optical conductivity and ARPES spectra of strong-coupling large polarons and its display in cuprates
Common approach is used to calculate band due to strong-coupling large
polaron (SCLP) photodissociation in ARPES and in optical conductivity (OC)
spectra. It is based on using the coherent-states representation for the phonon
field in SCLP. The calculated positions of both band maximums are universal
functions of one parameter - the SCLP binding energy Ep: ARPES band maximum
lies at binding energy about 3.2Ep; the OC band maximum is at the photon energy
about 4.2Ep. The half-widths of the bands are mainly determined by Ep and
slightly depend on Frohlich electron-phonon coupling constant: for its value
6-8 the ARPES band half-width is 1.7-1.3Ep and the OC band half-width is
2.8-2.2Ep. Using these results one can predict approximate position of ARPES
band maximum and half-width from the maximum of mid-IR OC band and vice versa.
Comparison of the results with experiments leads to a conclusion that
underdoped cuprates contain SCLPs with Ep=0.1-0.2 eV that is in good conformity
with the medium parameters in cuprates. The values of the polaron binding
energy determined from experimental ARPES and OC spectra of the same material
are in good conformity too: the difference between them is within 10 percent.Comment: 17 pages, 6 figure
Bi-Polaron and N-Polaron Binding Energies
The binding of polarons, or its absence, is an old and subtle topic. Here we
prove two things rigorously. First, the transition from many-body collapse to
the existence of a thermodynamic limit for N polarons occurs precisely at
U=2\alpha, where U is the electronic Coulomb repulsion and \alpha is the
polaron coupling constant. Second, if U is large enough, there is no
multi-polaron binding of any kind. Considering the known fact that there is
binding for some U>2\alpha, these conclusions are not obvious and their proof
has been an open problem for some time.Comment: 4 page
Optical properties of atomic Mott insulators: from slow light to dynamical Casimir effects
We theoretically study the optical properties of a gas of ultracold,
coherently dressed three-level atoms in a Mott insulator phase of an optical
lattice. The vacuum state, the band dispersion and the absorption spectrum of
the polariton field can be controlled in real time by varying the amplitude and
the frequency of the dressing beam. In the weak dressing regime, the system
shows unique ultra-slow light propagation properties without absorption. In the
presence of a fast time modulation of the dressing amplitude, we predict a
significant emission of photon pairs by parametric amplification of the
polaritonic zero-point fluctuations. Quantitative considerations on the
experimental observability of such a dynamical Casimir effect are presented for
the most promising atomic species and level schemes
A Theoretical Model for Gas Separation in a Glow Discharge: Cataphoresis
A theoretical model for transient and steady-state cataphoresis is developed starting with the macroscopic equations of continuity. After a brief breakdown period, the impurity ions are assumed to be closely coupled with their neutral counterparts. The basic assumptions in the model are that after breakdown, the level of ionization of the impurity, and the axial electric field remain constant; it is demonstrated that under these conditions a system involving rapid ionization-recombination reactions is equivalent to a system in which no reaction occurs, but in which the effective\u27\u27 ion mobility is a product of the true ion mobility and the fraction of impurity ionization. The influence of endbulbs commonly employed in experiments is analyzed and found to influence greatly the characteristic time required to reach steady state. Agreement is found between the model and available experimental data. Particular emphasis is placed upon mass spectrometer data reported by Matveeva, and by Beckey, Groth, and Welge; these data are for mixtures of rare gases and for mixtures of hydrogen and deuterium, and involve endbulbs. The ordinary diffusion case, associated with the collapse of the steady-state cataphoretic profile, is also analyzed for a system containing endbulbs
Directed current in the Holstein system
We propose a mechanism to rectify charge transport in the semiclassical
Holstein model. It is shown that localised initial conditions, associated with
a polaron solution, in conjunction with a nonreversion symmetric static
electron on-site potential constitute minimal prerequisites for the emergence
of a directed current in the underlying periodic lattice system. In particular,
we demonstrate that for unbiased spatially localised initial conditions,
violation of parity prevents the existence of pairs of counter-propagating
trajectories, thus allowing for a directed current despite the
time-reversibility of the equations of motion. Occurrence of long-range
coherent charge transport is demonstrated
Polarons and slow quantum phonons
We describe the formation and properties of Holstein polarons in the entire
parameter regime. Our presentation focuses on the polaron mass and radius,
which we obtain with an improved numerical technique. It is based on the
combination of variational exact diagonalization with an improved construction
of phonon states, providing results even for the strong coupling adiabatic
regime. In particular we can describe the formation of large and heavy
adiabatic polarons. A comparison of the polaron mass for the one and three
dimensional situation explains how the different properties in the static
oscillator limit determine the behavior in the adiabatic regime. The transport
properties of large and small polarons are characterized by the f-sum rule and
the optical conductivity. Our calculations are approximation-free and have
negligible numerical error. This allows us to give a conclusive and impartial
description of polaron formation. We finally discuss the implications of our
results for situations beyond the Holstein model.Comment: Final version, 10 pages, 10 figure
Electron locking in semiconductor superlattices
We describe a novel state of electrons and phonons arising in semiconductor
superlattices (SSL) due to strong electron-phonon interactions. These states
are characterized by a localization of phonons and a self-trapping or locking
of electrons in one or several quantum wells due to an additional,
deformational potential arising around these locking wells in SSL. The effect
is enhanced in a longitudinal magnetic field.
Using the tight-binding and adiabatic approximations the whole energy
spectrum of the self-trapped states is found and accurate, analytic expressions
are included for strong electron-phonon coupling. Finally, we discuss possible
experiments which may detect these predicted self-trapped states.Comment: 8 pages, 2 figures. Please note that the published article has the
title 'Electron locking in layered structures by a longitudinal magnetic
field
- …