42 research outputs found
Charge Pumping in Mesoscopic Systems coupled to a Superconducting Lead
We derive a general scattering-matrix formula for the pumped current through
a mesoscopic region attached to a normal and a superconducting lead. As
applications of this result we calculate the current pumped through (i) a pump
in a wire, (ii) a quantum dot in the Coulomb blockade regime, and (iii) a
ballistic double-barrier junction, all coupled to a superconducting lead.
Andreev reflection is shown to enhance the pumped current by up to a factor of
4 in case of equal coupling to the leads. We find that this enhancement can
still be further increased for slightly asymmetric coupling.Comment: 5 pages, 2 figure
Dynamic effect of phase conjugation on wave localization
We investigate what would happen to the time dependence of a pulse reflected
by a disordered single-mode waveguide, if it is closed at one end not by an
ordinary mirror but by a phase-conjugating mirror. We find that the waveguide
acts like a virtual cavity with resonance frequency equal to the working
frequency omega_0 of the phase-conjugating mirror. The decay in time of the
average power spectrum of the reflected pulse is delayed for frequencies near
omega_0. In the presence of localization the resonance width is
tau_s^{-1}exp(-L/l), with L the length of the waveguide, l the mean free path,
and tau_s the scattering time. Inside this frequency range the decay of the
average power spectrum is delayed up to times t simeq tau_s exp(L/l).Comment: 10 pages including 2 figure
Localization in a random phase-conjugating medium
We theoretically study reflection and transmission of light in a
one-dimensional disordered phase-conjugating medium. Using an invariant
imbedding approach a Fokker-Planck equation for the distribution of the probe
light reflectance and expressions for the average probabilities of reflection
and transmission are derived. A new crossover length scale for localization of
light is found, which depends on the competition between phase conjugation and
disorder. For weak disorder, our analytical results are in good agreement with
numerical simulations.Comment: RevTex, 4 pages, 4 figure
Andreev Bound States and Self-Consistent Gap Functions for SNS and SNSNS Systems
Andreev bound states in clean, ballistic SNS and SNSNS junctions are
calculated exactly and by using the Andreev approximation (AA). The AA appears
to break down for junctions with transverse dimensions chosen such that the
motion in the longitudinal direction is very slow. The doubly degenerate states
typical for the traveling waves found in the AA are replaced by two standing
waves in the exact treatment and the degeneracy is lifted.
A multiple-scattering Green's function formalism is used, from which the
states are found through the local density of states. The scattering by the
interfaces in any layered system of ballistic normal metals and clean
superconducting materials is taken into account exactly. The formalism allows,
in addition, for a self-consistent determination of the gap function. In the
numerical calculations the pairing coupling constant for aluminum is used.
Various features of the proximity effect are shown
Floquet scattering theory of quantum pumps
We develop the Floquet scattering theory for quantum mechanical pumping in
mesoscopic conductors. The nonequilibrium distribution function, the dc charge
and heat currents are investigated at arbitrary pumping amplitude and
frequency. For mesoscopic samples with discrete spectrum we predict a sign
reversal of the pumped current when the pump frequency is equal to the level
spacing in the sample. This effect allows to measure the phase of the
transmission coefficient through the mesoscopic sample. We discuss the
necessary symmetry conditions (both spatial and temporal) for pumping.Comment: 11 pages, 5 figure
Carbon nanotube-based quantum pump in the presence of superconducting lead
Parametric electron pump through superconductor-carbon-nanotube based
molecular devices was investigated. It is found that a dc current, which is
assisted by resonant Andreev reflection, can be pumped out from such molecular
device by a cyclic variation of two gate voltages near the nanotube. The pumped
current can be either positive or negative under different system parameters.
Due to the Andreev reflection, the pumped current has the double peak structure
around the resonant point. The ratio of pumped current of N-SWNT-S system to
that of N-SWNT-N system (I^{NS}/I^N) is found to approach four in the weak
pumping regime near the resonance when there is exactly one resonant level at
Fermi energy inside the energy gap. Numerical results confirm that in the weak
pumping regime the pumped current is proportional to the square of the pumping
amplitude V_p, but in the strong pumping regime the pumped current has the
linear relation with V_p. Our numerical results also predict that pumped
current can be obtained more easily by using zigzag tube than by using armchair
tube
Driven coherent oscillations of a single electron spin in a quantum dot
The ability to control the quantum state of a single electron spin in a
quantum dot is at the heart of recent developments towards a scalable
spin-based quantum computer. In combination with the recently demonstrated
exchange gate between two neighbouring spins, driven coherent single spin
rotations would permit universal quantum operations. Here, we report the
experimental realization of single electron spin rotations in a double quantum
dot. First, we apply a continuous-wave oscillating magnetic field, generated
on-chip, and observe electron spin resonance in spin-dependent transport
measurements through the two dots. Next, we coherently control the quantum
state of the electron spin by applying short bursts of the oscillating magnetic
field and observe about eight oscillations of the spin state (so-called Rabi
oscillations) during a microsecond burst. These results demonstrate the
feasibility of operating single-electron spins in a quantum dot as quantum
bits.Comment: Total 25 pages. 11 pages main text, 5 figures, 9 pages supplementary
materia
Report from the EPAA workshop: In vitro ADME in safety testing used by EPAA industry sectors
AbstractThere are now numerous in vitro and in silico ADME alternatives to in vivo assays but how do different industries incorporate them into their decision tree approaches for risk assessment, bearing in mind that the chemicals tested are intended for widely varying purposes? The extent of the use of animal tests is mainly driven by regulations or by the lack of a suitable in vitro model. Therefore, what considerations are needed for alternative models and how can they be improved so that they can be used as part of the risk assessment process? To address these issues, the European Partnership for Alternative Approaches to Animal Testing (EPAA) working group on prioritisation, promotion and implementation of the 3Rs research held a workshop in November, 2008 in Duesseldorf, Germany. Participants included different industry sectors such as pharmaceuticals, cosmetics, industrial- and agro-chemicals. This report describes the outcome of the discussions and recommendations (a) to reduce the number of animals used for determining the ADME properties of chemicals and (b) for considerations and actions regarding in vitro and in silico assays. These included: standardisation and promotion of in vitro assays so that they may become accepted by regulators; increased availability of industry in vivo kinetic data for a central database to increase the power of in silico predictions; expansion of the applicability domains of in vitro and in silico tools (which are not necessarily more applicable or even exclusive to one particular sector) and continued collaborations between regulators, academia and industry. A recommended immediate course of action was to establish an expert panel of users, developers and regulators to define the testing scope of models for different chemical classes. It was agreed by all participants that improvement and harmonization of alternative approaches is needed for all sectors and this will most effectively be achieved by stakeholders from different sectors sharing data
Sequence History Analysis (SHA) : Estimating the Effect of Past Trajectories on an Upcoming Event
In this article, we propose an innovative method which is a combination of Sequences Analysis and Event History Analysis. We called this method Sequence History Analysis (SHA). We start by identifying typical past trajectories of individuals over time by using Sequence Analysis. We then estimate the effect of these typical past trajectories on the event under study using discrete-time models. The aim of this approach is to estimate the effect of past trajectories on the chances of experiencing an event. We apply the proposed methodological approach to an original study of the effect of past childhood co-residence structures on the chances of leaving the parental home in Switzerland. The empirical research was based on the LIVES Cohort study, a panel survey that started in autumn 2013 in Switzerland. Analyses show that it is not only the occurrence of an event that increases the risk of experiencing another event, but also the order in which various states occurred. What is more, it seems that two features have a significant influence on departure from the parental home: the co-residence structures and the arrival or departure of siblings from the parental home