1,985 research outputs found
Theory of Polar Corrections to Donor Binding
We calculate the optical phonon correction to the binding energy of electrons
to donors in cubic materials. Previous theories calculated the Rydberg energy
reduced by the effective mass and the static dielectric function. They omitted
an important energy term from the long-range polarization of the ionized donor,
which vanishes for the neutral donor. They also omitted the donor-phonon
interaction. Including these terms yields a new formula for the donor binding
energy
Bulk metals with helical surface states
In the flurry of experiments looking for topological insulator materials, it
has been recently discovered that some bulk metals very close to topological
insulator electronic states, support the same topological surface states that
are the defining characteristic of the topological insulator. First observed in
spin-polarized ARPES in Sb (D. Hsieh et al. Science 323, 919 (2009)), the
helical surface states in the metallic systems appear to be robust to at least
mild disorder. We present here a theoretical investigation of the nature of
these "helical metals" - bulk metals with helical surface states. We explore
how the surface and bulk states can mix, in both clean and disordered systems.
Using the Fano model, we discover that in a clean system, the helical surface
states are \emph{not} simply absorbed by hybridization with a non-topological
parasitic metallic band. Instead, they are pushed away from overlapping in
momentum and energy with the bulk states, leaving behind a finite-lifetime
surface resonance in the bulk energy band. Furthermore, the hybridization may
lead in some cases to multiplied surface state bands, in all cases retaining
the helical characteristic. Weak disorder leads to very similar effects -
surface states are pushed away from the energy bandwidth of the bulk, leaving
behind a finite-lifetime surface resonance in place of the original surface
states
Quantum pump effect in one-dimensional systems of Dirac fermions
We investigate the behavior of the directed current in one-dimensional
systems of Dirac fermions driven by local periodic potentials in the forward as
well in backscattering channels. We treat the problem with Keldysh
non-equilibrium Green's function formalism. We present the exact solution for
the case of an infinite wire and show that in this case the dc current vanishes
identically. We also investigate a confined system consistent in an annular
arrangement coupled to a particle reservoir. We present a perturbative
treatment that allows for the analytical expressions of the dc current in the
lowest order of the amplitudes of the potential. We also present results
obtained from the exact numerical solution of the problem.Comment: 8 pages, 5 figure
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
Unconventional Hall effect in pnictides from interband interactions
We calculate the Hall transport in a multiband systems with a dominant
interband interaction between carriers having electron and hole character. We
show that this situation gives rise to an unconventional scenario, beyond the
Boltzmann theory, where the quasiparticle currents dressed by vertex
corrections acquire the character of the majority carriers. This leads to a
larger (positive or negative) Hall coefficient than what expected on the basis
of the carrier balance, with a marked temperature dependence. Our results
explain the puzzling measurements in pnictides and they provide a more general
framework for transport properties in multiband materials.Comment: 5 pages, 2 figure
Non-Markov dynamics and phonon decoherence of a double quantum dot charge qubit
In this paper we investigate decoherence times of a double quantum dot (DQD)
charge qubit due to it coupling with acoustic phonon baths. We individually
consider the acoustic piezoelectric as well as deformation coupling phonon
baths in the qubit environment. The decoherence times are calculated with two
kinds of methods. One of them is based on the qusiadiabatic propagator path
integral (QUAPI) and the other is based on Bloch equations, and two kinds of
results are compared. It is shown that the theoretical decoherence times of the
DQD charge qubit are shorter than the experimental reported results. It implies
that the phonon couplings to the qubit play a subordinate role, resulting in
the decoherence of the qubit.Comment: 5 pages, 4 figure
Phonon-assisted decoherence in the production of polarization-entangled photons in a single semiconductor quantum dot
We theoretically investigate the production of polarization-entangled photons
through the biexciton cascade decay in a single semiconductor quantum dot. In
the intermediate state the entanglement is encoded in the polarizations of the
first emitted photon and the exciton, where the exciton state can be
effectively ``measured'' by the solid state environment through the formation
of a lattice distortion. We show that the resulting loss of entanglement
becomes drastically enhanced if the phonons contributing to the lattice
distortion are subject to elastic scatterings at the device boundaries, which
might constitute a serious limitation for quantum-dot based entangled-photon
devices.Comment: 4 pages, 3 figure, to appear in Physical Review Letter
Suppression of electron relaxation and dephasing rates in quantum dots caused by external magnetic fields
An external magnetic field has been applied in laterally coupled dots (QDs)
and we have studied the QD properties related to charge decoherence. The
significance of the applied magnetic field to the suppression of
electron-phonon relaxation and dephasing rates has been explored. The coupled
QDs have been studied by varing the magnetic field and the interdot distance as
other system parameters. Our numerical results show that the electron
scattering rates are strongly dependent on the applied external magnetic field
and the details of the double QD configuration.Comment: 13 pages, 6 figure
Self-localized impurities embedded in a one dimensional Bose-Einstein condensate and their quantum fluctuations
We consider the self-localization of neutral impurity atoms in a
Bose-Einstein condensate in a 1D model. Within the strong coupling approach, we
show that the self-localized state exhibits parametric soliton behavior. The
corresponding stationary states are analogous to the solitons of non-linear
optics and to the solitonic solutions of the Schroedinger-Newton equation
(which appears in models that consider the connection between quantum mechanics
and gravitation). In addition, we present a Bogoliubov-de-Gennes formalism to
describe the quantum fluctuations around the product state of the strong
coupling description. Our fluctuation calculations yield the excitation
spectrum and reveal considerable corrections to the strong coupling
description. The knowledge of the spectrum allows a spectroscopic detection of
the impurity self-localization phenomenon.Comment: 7 pages, 5 figure
Spin-Forster transfer in optically excited quantum dots
The mechanisms of energy and spin transfer in quantum dot pairs coupled via
the Coulomb interaction are studied. Exciton transfer can be resonant or
phonon-assisted. In both cases, the transfer rates strongly depend on the
resonance conditions. The spin selection rules in the transfer process come
from the exchange and spin-orbit interactions. The character of energy
dissipation in spin transfer is different than that in the traditional spin
currents. The spin-dependent photon cross-correlation functions reflect the
exciton transfer process. In addition, a mathematical method to calculate
F\"orster transfer in crystalline nanostructures beyond the dipole-dipole
approximation is described.Comment: 22 pages, 10 figures, Phys. Rev. B, in pres
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