6,202 research outputs found
Twisted-light-induced optical transitions in semiconductors: Free-carrier quantum kinetics
We theoretically investigate the interband transitions and quantum kinetics
induced by light carrying orbital angular momentum, or twisted light, in bulk
semiconductors. We pose the problem in terms of the Heisenberg equations of
motion of the electron populations, and inter- and intra-band coherences. Our
theory extends the free-carrier Semiconductor Bloch Equations to the case of
photo-excitation by twisted light. The theory is formulated using cylindrical
coordinates, which are better suited to describe the interaction with twisted
light than the usual cartesian coordinates used to study regular optical
excitation. We solve the equations of motion in the low excitation regime, and
obtain analytical expressions for the coherences and populations; with these,
we calculate the orbital angular momentum transferred from the light to the
electrons and the paramagnetic and diamagnetic electric current densities.Comment: 11 pages, 3 figure
Pseudospin dynamics in multimode polaritonic Josephson junctions
We analyzed multimode Josephson junctions with exciton-polaritons
(polaritonic Josephson junctions) when several coupling mechanisms of
fundamental and excited states are present. The applied method is based on
Keldysh-Green function formalism and takes into account polariton pseudospin.
We found that mean value of circular polarization degree in intrinsic Josephson
oscillations and microscopic quantum self-trapping follow an oscillator
behavior whose renormalizes due to intermode interactions. The effect of an
additional transfer of particles over junction barrier occurring in multimode
approximation in combination with common Josephson tunneling is discussed in
regime of dynamical separation of two polarizations.Comment: 12 pages, 4 figure
Excitonic Dynamical Franz-Keldysh Effect
The Dynamical Franz-Keldysh Effect is exposed by exploring near-bandgap
absorption in the presence of intense THz electric fields. It bridges the gap
between the DC Franz- Keldysh effect and multi-photon absorption and competes
with the THz AC Stark Effect in shifting the energy of the excitonic resonance.
A theoretical model which includes the strong THz field non-perturbatively via
a non-equilibrium Green Functions technique is able to describe the Dynamical
Franz-Keldysh Effect in the presence of excitonic absorption.Comment: 4 pages in revtex with 5 figures included using epsf. Submitted to
Physical Review Letter
Molecular junctions in the Coulomb blockade regime: rectification and nesting
Quantum transport through single molecules is very sensitive to the strength
of the molecule-electrode contact. Here, we investigate the behavior of a model
molecular junction weakly coupled to external electrodes in the case where
charging effects do play an important role (Coulomb blockade regime). As a
minimal model we consider a molecular junction with two spatially separated
donor and acceptor sites. Depending on their mutual coupling to the electrodes,
the resulting transport observables show well defined features such as
rectification effects in the I-V characteristics and nesting of the stability
diagrams. To be able to accomplish these results, we have developed a theory
which allows to explore the charging regime via the nonequilibrium Green
function formalism parallel to the widely used master equation technique. Our
results, beyond their experimental relevance, offer a transparent framework for
the systematic and modular inclusion of a richer physical phenomenology
Using spin bias to manipulate and measure quantum spin in quantum dots
A double-quantum-dot coupled to electrodes with spin-dependent splitting of
chemical potentials (spin bias) is investigated theoretically by means of the
Green's functions formalism. By applying a large spin bias, the quantum spin in
a quantum dot (the dot 1) can be manipulated in a fully electrical manner. To
noninvasively monitor the manipulation of the quantum spin in the dot 1, it is
proposed that the second quantum dot (the dot 2) is weakly coupled to the dot
1. In the presence of the exchange interaction between the two dots, the
polarized spin in the dot 1 behaves like an effective magnetic field and weakly
polarizes the spin in the nearby quantum dot 2. By applying a very small spin
bias to the dot 2, the spin-dependent transport through the dot 2 can be
probed, allowing the spin polarization in the dot 1 to be identified
nondestructively. These two steps form a complete scheme to manipulate a
trapped spin while permitting this manipulation to be monitored in the
double-dot system using pure electric approaches
Nonequilibrium fluctuation-dissipation relations for one- and two-particle correlation functions in steady-state quantum transport
We study the non-equilibrium (NE) fluctuation-dissipation (FD) relations in
the context of quantum thermoelectric transport through a two-terminal
nanodevice in the steady-state. The FD relations for the one- and two-particle
correlation functions are derived for a model of the central region consisting
of a single electron level. Explicit expressions for the FD relations of the
Green's functions (one-particle correlations) are provided. The FD relations
for the current-current and charge-charge (two-particle) correlations are
calculated numerically. We use self-consistent NE Green's functions
calculations to treat the system in the absence and in the presence of
interaction (electron-phonon) in the central region. We show that, for this
model, there is no single universal FD theorem for the NE steady state. There
are different FD relations for each different class of problems. We find that
the FD relations for the one-particle correlation function are strongly
dependent on both the NE conditions and the interactions, while the FD
relations of the current-current correlation function are much less dependent
on the interaction. The latter property suggests interesting applications for
single-molecule and other nanoscale transport experiments.Comment: This revised version is now accepted for publication in the Journal
of Chemical Physics (March 2014). arXiv admin note: text overlap with
arXiv:1305.507
Thermoelectric and thermal rectification properties of quantum dot junctions
The electrical conductance, thermal conductance, thermal power and figure of
merit (ZT) of semiconductor quantum dots (QDs) embedded into an insulator
matrix connected with metallic electrodes are theoretically investigated in the
Coulomb blockade regime. The multilevel Anderson model is used to simulate the
multiple QDs junction system. The charge and heat currents in the sequential
tunneling process are calculated by the Keldysh Green function technique. In
the linear response regime the ZT values are still very impressive in the small
tunneling rates case, although the effect of electron Coulomb interaction on ZT
is significant. In the nonlinear response regime, we have demonstrated that the
thermal rectification behavior can be observed for the coupled QDs system,
where the very strong asymmetrical coupling between the dots and electrodes,
large energy level separation between dots and strong interdot Coulomb
interactions are required.Comment: 8 page and 14 figure
Kinetics of four-wave mixing for a 2D magneto-plasma in strong magnetic fields
We investigate the femtosecond kinetics of an optically excited 2D
magneto-plasma at intermediate and high densities under a strong magnetic field
perpendicular to the quantum well (QW). We assume an additional weak lateral
confinement which lifts the degeneracy of the Landau levels partially. We
calculate the femtosecond dephasing and relaxation kinetics of the laser pulse
excited magneto-plasma due to bare Coulomb potential scattering, because
screening is under these conditions of minor importance. In particular the
time-resolved and time-integrated four-wave mixing (FWM) signals are calculated
by taking into account three Landau subbands in both the valance and the
conduction band assuming an electron-hole symmetry. The FWM signals exhibit
quantum beats mainly with twice the cyclotron frequency. Contrary to general
expectations, we find no pronounced slowing down of the dephasing with
increasing magnetic field. On the contrary, one obtains a decreasing dephasing
time because of the increase of the Coulomb matrix elements and the number of
states in a given Landau subband. In the situation when the loss of scattering
channels exceeds these increasing effects, one gets a slight increase at the
dephasing time. However, details of the strongly modulated scattering kinetics
depend sensitively on the detuning, the plasma density, and the spectral pulse
width relative to the cyclotron frequency.Comment: 13 pages, in RevTex format, 10 figures, Phys. Rev B in pres
On simulation of local fluxes in molecular junctions
We present a pedagogical review of current density simulation in molecular
junction models indicating its advantages and deficiencies in analysis of local
junction transport characteristics. In particular, we argue that current
density is a universal tool which provides more information than traditionally
simulated bond currents, especially when discussing inelastic processes.
However, current density simulations are sensitive to choice of basis and
electronic structure method. We note that discussing local current conservation
in junctions one has to account for source term caused by open character of the
system and intra-molecular interactions. Our considerations are illustrated
with numerical simulations of a benzenedithiol molecular junction.Comment: 10 pages, 6 figure
A complete quasiclassical map for the dynamics of interacting fermions
We present a strategy for mapping the dynamics of a fermionic quantum system
to a set of classical dynamical variables. The approach is based on imposing
the correspondence relation between the commutator and the Poisson bracket,
preserving Heisenberg's equation of motion for one-body operators. In order to
accommodate the effect of two-body terms, we further impose quantization on the
spin-dependent occupation numbers in the classical equations of motion, with a
parameter that is determined self-consistently. Expectation values for
observables are taken with respect to an initial quasiclassical distribution
that respects the original quantization of the occupation numbers. The proposed
classical map becomes complete under the evolution of quadratic Hamiltonians
and is extended for all even order observables. We show that the map provides
an accurate description of the dynamics for an interacting quantum impurity
model in the coulomb blockade regime, at both low and high temperatures. The
numerical results are aided by a novel importance sampling scheme that employs
a reference system to reduce significantly the sampling effort required to
converge the classical calculations
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