20,127 research outputs found
Quantum quenches and driven dynamics in a single-molecule device
The nonequilibrium dynamics of molecular devices is studied in the framework
of a generic model for single-molecule transistors: a resonant level coupled by
displacement to a single vibrational mode. In the limit of a broad level and in
the vicinity of the resonance, the model can be controllably reduced to a form
quadratic in bosonic operators, which in turn is exactly solvable. The response
of the system to a broad class of sudden quenches and ac drives is thus
computed in a nonperturbative manner, providing an asymptotically exact
solution in the limit of weak electron-phonon coupling. From the analytic
solution we are able to (1) explicitly show that the system thermalizes
following a local quantum quench, (2) analyze in detail the time scales
involved, (3) show that the relaxation time in response to a quantum quench
depends on the observable in question, and (4) reveal how the amplitude of
long-time oscillations evolves as the frequency of an ac drive is tuned across
the resonance frequency. Explicit analytical expressions are given for all
physical quantities and all nonequilibrium scenarios under study.Comment: 23 pages, 13 figure
Quantum phase transition in the Dicke model with critical and non-critical entanglement
We study the quantum phase transition of the Dicke model in the classical
oscillator limit, where it occurs already for finite spin length. In contrast
to the classical spin limit, for which spin-oscillator entanglement diverges at
the transition, entanglement in the classical oscillator limit remains small.
We derive the quantum phase transition with identical critical behavior in the
two classical limits and explain the differences with respect to quantum
fluctuations around the mean-field ground state through an effective model for
the oscillator degrees of freedom. With numerical data for the full quantum
model we study convergence to the classical limits. We contrast the classical
oscillator limit with the dual limit of a high frequency oscillator, where the
spin degrees of freedom are described by the Lipkin-Meshkov-Glick model. An
alternative limit can be defined for the Rabi case of spin length one-half, in
which spin frequency renormalization replaces the quantum phase transition.Comment: 1o pages, 10 figures, published versio
Influence of Anomalous Dispersion on Optical Characteristics of Quantum Wells
Frequency dependencies of optical characteristics (reflection, transmission
and absorption of light) of a quantum well are investigated in a vicinity of
interband resonant transitions in a case of two closely located excited energy
levels. A wide quantum well in a quantizing magnetic field directed normally to
the quantum-well plane, and monochromatic stimulating light are considered.
Distinctions between refraction coefficients of barriers and quantum well, and
a spatial dispersion of the light wave are taken into account. It is shown that
at large radiative lifetimes of excited states in comparison with nonradiative
lifetimes, the frequency dependence of the light reflection coefficient in the
vicinity of resonant interband transitions is defined basically by a curve,
similar to the curve of the anomalous dispersion of the refraction coefficient.
The contribution of this curve weakens at alignment of radiative and
nonradiative times, it is practically imperceptible at opposite ratio of
lifetimes . It is shown also that the frequency dependencies similar to the
anomalous dispersion do not arise in transmission and absorption coefficients.Comment: 10 pages, 6 figure
Transmission of a Symmetric Light Pulse through a Wide QW
The reflection, transmission and absorption of a symmetric electromagnetic
pulse, which carrying frequency is close to the frequency of an interband
transition in a QW (QW), are obtained. The energy levels of a QW are assumed
discrete, one exited level is taken into account. The case of a wide QW is
considered when a length of the pulse wave, appropriate to the carrying
frequency, is comparable to the QW's width. In figures the time dependencies of
the dimensionless reflection, absorption are transmission are represented. It
is shown, that the spatial dispersion and a distinction in refraction indexes
influence stronger reflection.Comment: 8 pages,8 figures with caption
Green's and spectral functions of the small Frolich polaron
According to recent Quantum Monte Carlo simulations the small polaron theory
is practically exact in a wide range of the long-range (Frohlich)
electron-phonon coupling and adiabatic ratio. We apply the Lang-Firsov
transformation to convert the strong-coupling term in the Hamiltonian into the
form of an effective hopping integral and derive the single-particle Green's
function describing propagation of the small Frohlich polaron. One and two
dimensional spectral functions are studied by expanding the Green's function
perturbatively. Numerical calculations of the spectral functions are produced.
Remarkably, the coherent spectral weight (Z) and effective mass (Z')
renormalisation exponents are found to be different with Z'>>Z, which can
explain a small coherent spectral weight and a relatively moderate mass
enhancement in oxides.Comment: RevTeX, 5 pages, 2 postscript figures, LaTeX processing problems
correcte
Elastic Light Scattering by Semiconductor Quantum Dots
Elastic light scattering by low-dimensional semiconductor objects is
investigated theoretically. The differential cross section of resonant light
scattering on excitons in quantum dots is calculated. The polarization and
angular distribution of scattered light do not depend on the quantum-dot form,
sizes and potential configuration if light wave lengths exceed considerably the
quantum-dot size. In this case the magnitude of the total light scattering
cross section does not depend on quantum-dot sizes. The resonant total light
scattering cross section is about a square of light wave length if the exciton
radiative broadening exceeds the nonradiative broadening. Radiative broadenings
are calculated
Principals of the theory of light reflection and absorption by low-dimensional semiconductor objects in quantizing magnetic fields at monochromatic and pulse excitations
The bases of the theory of light reflection and absorption by low-dimensional
semiconductor objects (quantum wells, wires and dots) at both monochromatic and
pulse irradiations and at any form of light pulses are developed. The
semiconductor object may be placed in a stationary quantizing magnetic field.
As an example the case of normal light incidence on a quantum well surface is
considered. The width of the quantum well may be comparable to the light wave
length and number of energy levels of electronic excitations is arbitrary. For
Fourier-components of electric fields the integral equation (similar to the
Dyson-equation) and solutions of this equation for some individual cases are
obtained.Comment: 14 page
Many-Polaron Effects in the Holstein Model
We derive an effective polaronic interaction Hamiltonian, {\it exact to
second order in perturbation}, for the spinless one-dimensional Holstein model.
The small parameter is given by the ratio of the hopping term () to the
polaronic energy () in all the region of validity for our
perturbation; however, the exception being the regime of extreme
anti-adiabaticity () and small electron-phonon coupling () where the small parameter is . We map our polaronic
Hamiltonian onto a next-to-nearest-neighbor interaction anisotropic Heisenberg
spin model. By studying the mass gap and the power-law exponent of the
spin-spin correlation function for our Heisenberg spin model, we analyze the
Luttinger liquid to charge-density-wave transition at half-filling in the
effective polaronic Hamiltonian. We calculate the structure factor at all
fillings and find that the spin-spin correlation length decreases as one
deviates from half-filling. We also extend our derivation of polaronic
Hamiltonian to -dimensions.Comment: Content changed. Accepted in Phys. Rev.
Alternative approach to computing transport coefficients: application to conductivity and Hall coefficient of hydrogenated amorphous silicon
We introduce a theoretical framework for computing transport coefficients for
complex materials. As a first example, we resolve long-standing inconsistencies
between experiment and theory pertaining to the conductivity and Hall mobility
for amorphous silicon and show that the Hall sign anomaly is a consequence of
localized states. Next, we compute the AC conductivity of amorphous
polyanaline. The formalism is applicable to complex materials involving defects
and band-tail states originating from static topological disorder and extended
states. The method may be readily integrated with current \textit{ab initio}
methods.Comment: 4 pages, 2 figures, submitted to Phys. Rev. Let
Profile alterations of a symmetrical light pulse coming through a quantum well
The theory of a response of a two-energy-level system, irradiated by
symmetrical light pulses, has been developed.(Suchlike electronic system
approximates under the definite conditions a single ideal quantum well (QW) in
a strong magnetic field {\bf H}, directed perpendicularly to the QW's plane, or
in magnetic field absence.) The general formulae for the time-dependence of
non-dimensional reflection {\cal R}(t), absorption {\cal A}(t) and transmission
{\cal T}(t) of a symmetrical light pulse have been obtained. It has been shown
that the singularities of three types exist on the dependencies {\cal R}(t),
{\cal A}(t), {\cal T}(t). The oscillating time dependence of {\cal R}(t), {\cal
A}(t), {\cal T}(t) on the detuning frequency \Delta\omega=\omega_l-\omega_0
takes place. The oscillations are more easily observable when
\Delta\omega\simeq\gamma_l. The positions of the total absorption, reflection
and transparency singularities are examined when the frequency \omega_l is
detuned.Comment: 9 pages, 13 figures with caption
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