1,501 research outputs found
Quantum versus classical counting in nonMarkovian master equations
We discuss the description of full counting statistics in quantum transport
with a nonMarkovian master equation. We focus on differences arising from
whether charge is considered as a classical or a quantum degree of freedom.
These differences manifest themselves in the inhomogeneous term of the master
equation which describes initial correlations. We describe the influence on
current and in particular, the finite-frequency shotnoise. We illustrate these
ideas by studying transport through a quantum dot and give results that include
both sequential and cotunneling processes. Importantly, the noise spectra
derived from the classical description are essentially frequency-independent
and all quantum noise effects are absent. These effects are fully recovered
when charge is considered as a quantum degree of freedom.Comment: 12 pages; 3 figure
The puzzling interpretation of NIR indices: The case of NaI2.21
We present a detailed study of the Na I line strength index centered in the
-band at , {\AA} (NaI2.21 hereafter) relying on different samples of
early-type galaxies. Consistent with previous studies, we find that the
observed line strength indices cannot be fit by state-of-art scaled-solar
stellar population models, even using our newly developed models in the NIR.
The models clearly underestimate the large NaI2.21 values measured for most
early-type galaxies. However, we develop a Na-enhanced version of our newly
developed models in the NIR, which - together with the effect of a bottom-heavy
initial mass function - yield NaI2.21 indices in the range of the observations.
Therefore, we suggest a scenario in which the combined effect of [Na/Fe]
enhancement and a bottom-heavy initial mass function are mainly responsible for
the large NaI2.21 indices observed for most early-type galaxies. To a smaller
extent, also [C/Fe] enhancement might contribute to the large observed NaI2.21
values.Comment: 13 pages, 4 figures, accepted for publication in MNRA
Entanglement between charge qubits induced by a common dissipative environment
We study entanglement generation between two charge qubits due to the strong
coupling with a common bosonic environment (Ohmic bath). The coupling to the
boson bath is a source of both quantum noise (leading to decoherence) and an
indirect interaction between qubits. As a result, two effects compete as a
function of the coupling strength with the bath: entanglement generation and
charge localization induced by the bath. These two competing effects lead to a
non-monotonic behavior of the concurrence as a function of the coupling
strength with the bath. As an application, we present results for charge qubits
based on double quantum dots.Comment: 9 pages, 7 figure
Hydrodynamic Aspects and Correlations for the Design of Draft-Tube Conical Spouted Beds
A study has been carried out on the hydrodynamics of conical spouted beds with draft tube. Correlations have been proposed for calculating minimum spouting velocity, operating pressure drop and peak pressure drop as functions of dimensionless module that take into account geometric factors, particle characteristics and operating conditions
Origin of the anomalies: the modified Heisenberg equation
The origin of the anomalies is analyzed. It is shown that they are due to the
fact that the generators of the symmetry do not leave invariant the domain of
definition of the Hamiltonian and then a term, normally forgotten in the
Heisenberg equation, gives an extra contribution responsible for the non
conservation of the charges. This explanation is equivalent to that of the
Fujikawa in the path integral formalism. Finally, this approach is applied to
the conformal symmetry breaking in two-dimensional quantum mechanics.Comment: 7 pages, LaTe
Non-perturbative electron dynamics in crossed fields
Intense AC electric fields on semiconductor structures have been studied in
photon-assisted tunneling experiments with magnetic field applied either
parallel (B_par) or perpendicular (B_per) to the interfaces. We examine here
the electron dynamics in a double quantum well when intense AC electric fields
F, and tilted magnetic fields are applied simultaneously. The problem is
treated non-perturbatively by a time-dependent Hamiltonian in the effective
mass approximation, and using a Floquet-Fourier formalism. For B_par=0, the
quasi-energy spectra show two types of crossings: those related to different
Landau levels, and those associated to dynamic localization (DL), where the
electron is confined to one of the wells, despite the non-negligible tunneling
between wells. B_par couples parallel and in-plane motions producing
anti-crossings in the spectrum. However, since our approach is
non-perturbative, we are able to explore the entire frequency range. For high
frequencies, we reproduce the well known results of perfect DL given by zeroes
of a Bessel function. We find also that the system exhibits DL at the same
values of the field F, even as B_par non-zero, suggesting a hidden dynamical
symmetry in the system which we identify with different parity operations. The
return times for the electron at various values of field exhibit interesting
and complex behavior which is also studied in detail. We find that smaller
frequencies shifts the DL points to lower field F, and more importantly, yields
poorer localization by the field. We analyze the explicit time evolution of the
system, monitoring the elapsed time to return to a given well for each Landau
level, and find non-monotonic behavior for decreasing frequencies.Comment: REVTEX4 + 11 eps figs, submitted to Phys. Rev.
Finite-frequency counting statistics of electron transport: Markovian Theory
We present a theory of frequency-dependent counting statistics of electron
transport through nanostructures within the framework of Markovian quantum
master equations. Our method allows the calculation of finite-frequency current
cumulants of arbitrary order, as we explicitly show for the second- and
third-order cumulants. Our formulae generalize previous zero-frequency
expressions in the literature and can be viewed as an extension of MacDonald's
formula beyond shot noise. When combined with an appropriate treatment of
tunneling, using, e.g. Liouvillian perturbation theory in Laplace space, our
method can deal with arbitrary bias voltages and frequencies, as we illustrate
with the paradigmatic example of transport through a single resonant level
model. We discuss various interesting limits, including the recovery of the
fluctuation-dissipation theorem near linear response, as well as some drawbacks
inherent of the Markovian description arising from the neglect of quantum
fluctuations.Comment: Accepted in New Journal of Physics. Updated tex
Control of Dephasing and Phonon Emission in Coupled Quantum Dots
We predict that phonon subband quantization can be detected in the non-linear
electron current through double quantum dot qubits embedded into nano-size
semiconductor slabs, acting as phonon cavities. For particular values of the
dot level splitting , piezo-electric or deformation potential
scattering is either drastically reduced as compared to the bulk case, or
strongly enhanced due to phonon van Hove singularities. By tuning via
gate voltages, one can either control dephasing, or strongly increase emission
into phonon modes with characteristic angular distributions.Comment: 4 pages, 3 figures, accepted for publication as Rapid Comm. in Phys.
Rev.
Truncation method for Green's functions in time-dependent fields
We investigate the influence of a time dependent, homogeneous electric field
on scattering properties of non-interacting electrons in an arbitrary static
potential. We develop a method to calculate the (Keldysh) Green's function in
two complementary approaches. Starting from a plane wave basis, a formally
exact solution is given in terms of the inverse of a matrix containing
infinitely many 'photoblocks' which can be evaluated approximately by
truncation. In the exact eigenstate basis of the scattering potential, we
obtain a version of the Floquet state theory in the Green's functions language.
The formalism is checked for cases such as a simple model of a double barrier
in a strong electric field. Furthermore, an exact relation between the
inelastic scattering rate due to the microwave and the AC conductivity of the
system is derived which in particular holds near or at a metal-insulator
transition in disordered systems.Comment: to appear in Phys. Rev. B., 21 pages, 3 figures (ps-files
Photon-Assisted Transport Through Ultrasmall Quantum Dots: Influence of Intradot Transitions
We study transport through one or two ultrasmall quantum dots with discrete
energy levels to which a time-dependent field is applied (e.g., microwaves).
The AC field causes photon-assisted tunneling and also transitions between
discrete energy levels of the dot. We treat the problem by introducing a
generalization of the rotating-wave approximation to arbitrarily many levels.
We calculate the dc-current through one dot and find satisfactory agreement
with recent experiments by Oosterkamp et al. . In addition, we propose a novel
electron pump consisting of two serially coupled single-level quantum dots with
a time-dependent interdot barrier.Comment: 16 pages, Revtex, 10 eps-figure
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