1,834 research outputs found
Variational approach to transport in quantum dots
We have derived a variational principle that defines the nonequilibrium
steady-state transport across a correlated impurity mimicking, e.g., a quantum
dot coupled to biased leads. This variational principle has been specialized to
a Gutzwiller's variational space, and applied to the study of the simple
single-orbital Anderson impurity model at half filling, finding a good
qualitative accord with the observed behavior in quantum dots for the expected
regime of values of the bias. Beyond the purely theoretical interest in the
formal definition of a variational principle in a nonequilibrium problem, the
particular methods proposed have the important advantage to be simple and
flexible enough to deal with more complicated systems and variational spaces.Comment: 15 pages, 4 figure
Fano-Kondo effect in side-coupled double quantum dots at finite temperatures and the importance of the two-stage Kondo screening
We study the zero-bias conductance through the system of two quantum dots,
one of which is embedded directly between the source and drain electrodes,
while the second dot is side-coupled to the first one through a tunneling
junction. Modeling the system using the two-impurity Anderson model, we compute
the temperature-dependence of the conductance in various parameter regimes
using the numerical renormalization group. We consider the non-interacting
case, where we study the extent of the departure from the conventional Fano
resonance line shape at finite temperatures, and the case where the embedded
and/or the side-coupled quantum dot is interacting, where we study the
consequences of the coexistence of the Kondo and Fano effects. If the
side-coupled dot is very weakly interacting, the occupancy changes by two when
the on-site energy crosses the Fermi level and a Fano-resonance-like shape is
observed. If the interaction on the side-coupled dot is sizeable, the occupancy
changes only by one and a very different line-shape results, which is strongly
and characteristically temperature dependent. These results suggest an
intriguing alternative interpretation of the recent experimental results study
of the transport properties of the side-coupled double quantum dot [Sasaki et
al., Phys. Rev. Lett. 103, 266806 (2009)]: the observed Fano-like conductance
anti-resonance may, in fact, result from the two-stage Kondo effect in the
regime where the experimental temperature is between the higher and the lower
Kondo temperature.Comment: 9 pages, 11 figures. In V2: updated references, 3 new figures,
additional discussio
Shifts in ownership toward high-powered motorcycles and its effects on public health
Objectives. We assessed whether policies designed to safeguard young motorcyclists would be effective given shifts in ownership toward high-powered motorcycles. Methods. We investigated population-wide motor vehicle driver and motorcyclist casualties (excluding passengers) recorded in Britain between 2002 and 2009. To adjust for exposure and measure individual risk, we used the estimated number of trips of motorcyclists and drivers, which had been collected as part of a national travel survey. Results. Motorcyclists were 76 times more likely to be killed than were drivers for every trip. Older motorcyclist age—strongly linked to experience, skill set,and riding behavior—did not abate the risks of high-powered motorcycles. Older motorcyclists made more trips on high-powered motorcycles. Conclusions: Tighter engine size restrictions would help reduce the use of high-powered motorcycles. Policymakers should introduce health warnings on the risks of high-powered motorcycles and the benefits of safety equipment
Mixed-state aspects of an out-of-equilibrium Kondo problem in a quantum dot
We reexamine basic aspects of a nonequilibrium steady state in the Kondo
problem for a quantum dot under a bias voltage using a reduced density matrix,
which is obtained in the Fock space by integrating out one of the two
conduction channels. The integration has been carried out by discretizing the
conduction channels preserving the two-fold degeneracy due to the left-going
and right-going scattering states. The remaining subspace is described by a
single-channel Anderson model, and the statistical weight is determined by the
reduced density matrix. In the noninteracting case, it can be constructed as
the mixed states that show a close similarity to the high-temperature
distribution in equilibrium. Specifically, if the system has an inversion
symmetry, the one-particle states in an energy window between the two chemical
potentials \mu_R and \mu_L are occupied, or unoccupied, completely at random
with an equal weight. The Coulomb interaction preserves these aspects, and the
correlation functions can be expressed in a Lehmann-representation form using
the mixed-state statistical weight.Comment: 8 pages, 3 figure
The Influence of Interference on the Kondo Effect in a Quantum Dot
We study the Kondo effect in a model system of a quantum dot embedded in an
Aharanov-Bohm ring connected to two leads. By transforming to the scattering
basis of the direct inter-lead tunneling, we are able to describe precisely how
the Kondo screening of the dot spin occurs. We calculate the Kondo temperature
and zero-temperature conductance and find that both are influenced by the
Aharanov-Bohm ring as well as the electron density in the leads. We also
calculate the form of an additional potential scattering term that arises at
low energies due to the breaking of particle-hole symmetry. Many of our results
are supported by numerical analysis using the numerical renormalization group.Comment: 24 pages, 18 figure
Do adverts increase the probability of finding online cognitive behavioural therapy for depression? Cross-sectional study
Objective To estimate the effect of online adverts on the probability of finding online cognitive behavioural therapy (CBT) for depression.
Design Exploratory online cross-sectional study of search experience of people in the UK with depression in 2011. (1) The authors identified the search terms over 6 months entered by users who subsequently clicked on the advert for online help for depression. (2) A panel of volunteers across the UK recorded websites presented by normal Google search for the term ‘depression’. (iii) The authors examined these websites to estimate probabilities of knowledgeable and naive internet users finding online CBT and the improved probability by addition of a Google advert.
Participants (1) 3868 internet users entering search terms related to depression into Google. (2) Panel, recruited online, of 12 UK participants with an interest in depression.
Main outcome measures Probability of finding online CBT for depression with/without an advert.
Results The 3868 users entered 1748 different search terms but the single keyword ‘depression’ resulted in two-thirds of the presentations of, and over half the ‘clicks’ on, the advert. In total, 14 different websites were presented to our panel in the first page of Google results for ‘depression’. Four of the 14 websites had links enabling access to online CBT in three clicks for knowledgeable users. Extending this approach to the 10 most frequent search terms, the authors estimated probabilities of finding online CBT as 0.29 for knowledgeable users and 0.006 for naive users, making it unlikely CBT would be found. Adding adverts that linked directly to online CBT increased the probabilities to 0.31 (knowledgeable) and 0.02 (naive).
Conclusions In this case, online CBT was not easy to find and online adverts substantially increased the chance for naive users. Others could use this approach to explore additional impact before committing to long-term Google AdWords advertising budgets
Photoluminiscence of a quantum dot hybridized with a continuum
We calculate the intensity of photon emission from a trion in a single
quantum dot, as a function of energy and gate voltage, using the impurity
Anderson model and variational wave functions. Assuming a flat density of
conduction states and constant hybridization energy, the results agree with the
main features observed in recent experiments: non-monotonic dependence of the
energy on gate voltage, non-Lorentzian line shapes, and a line width that
increases near the regions of instability of the single electron final state to
occupations zero or two.Comment: 4 pages, 3 figures, Journal-ref adde
Kondo screening cloud in a one dimensional wire: Numerical renormalization group study
We study the Kondo model --a magnetic impurity coupled to a one dimensional
wire via exchange coupling-- by using Wilson's numerical renormalization group
(NRG) technique. By applying an approach similar to which was used to compute
the two impurity problem we managed to improve the bad spatial resolution of
the numerical renormalization group method. In this way we have calculated the
impurity spin - conduction electron spin correlation function which is a
measure of the Kondo compensation cloud whose existence has been a long
standing problem in solid state physics. We also present results on the
temperature dependence of the Kondo correlations.Comment: published versio
Suppression of Kondo-assisted co-tunneling in a spin-1 quantum dot with Spin-Orbit interaction
Kondo-type zero-bias anomalies have been frequently observed in quantum dots
occupied by two electrons and attributed to a spin-triplet configuration that
may become stable under particular circumstances. Conversely, zero-bias
anomalies have been so far quite elusive when quantum dots are occupied by an
even number of electrons greater than two, even though a spin-triplet
configuration is more likely to be stabilized there than for two electrons. We
propose as an origin of this phenomenon the spin-orbit interaction, and we show
how it profoundly alters the conventional Kondo screening scenario in the
simple case of a laterally confined quantum dot with four electrons.Comment: 5 pages, 3 figures, submitted 05May201
Numerical renormalization group study of two-channel three-impurity triangular clusters
We study triangular clusters of three spin-1/2 Kondo or Anderson impurities
that are coupled to two conduction leads. In the case of Kondo impurities, the
model takes the form of an antiferromagnetic Heisenberg ring with Kondo-like
exchange coupling to continuum electrons. We show that this model exhibits many
types of the behavior found in various simpler one and two-impurity models,
thereby enabling the study of crossovers between a number of Fermi-liquid (FL)
and non-Fermi-liquid (NFL) fixed points. In particular, we explore a direct
crossover between the two-impurity Kondo-model NFL fixed point and the
two-channel Kondo-model NFL fixed point. We show that the concept of the
two-stage Kondo effect applies even in the case when the first-stage Kondo
state is of NFL type. In the case of Anderson impurities, we consider the
transport properties of three coupled quantum dots. This class of models
includes as limiting cases the familiar serial double quantum dot and triple
quantum dot nanostructures. By extracting the quasiparticle scattering phase
shifts, we compute the low-temperature conductance as a function of the
inter-impurity tunneling-coupling. We point out that due to the existence of
exponentially low temperature scales, there is a parameter range where the
stable "zero-temperature" fixed point is essentially never reached (not even in
numerical renormalization group calculations). The "zero-temperature"
conductance is then of no interest and it may only be meaningful to compute the
conductance at finite temperature. This illustrates the perils of studying the
conductance in the ground state and considering thermal fluctuations only as a
small correction.Comment: 12 pages, 13 figure
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