168 research outputs found
Semiclassical approach to the ac-conductance of chaotic cavities
We address frequency-dependent quantum transport through mesoscopic
conductors in the semiclassical limit. By generalizing the trajectory-based
semiclassical theory of dc quantum transport to the ac case, we derive the
average screened conductance as well as ac weak-localization corrections for
chaotic conductors. Thereby we confirm respective random matrix results and
generalize them by accounting for Ehrenfest time effects. We consider the case
of a cavity connected through many leads to a macroscopic circuit which
contains ac-sources. In addition to the reservoir the cavity itself is
capacitively coupled to a gate. By incorporating tunnel barriers between cavity
and leads we obtain results for arbitrary tunnel rates. Finally, based on our
findings we investigate the effect of dephasing on the charge relaxation
resistance of a mesoscopic capacitor in the linear low-frequency regime
Ehrenfest-time dependence of counting statistics for chaotic ballistic systems
Transport properties of open chaotic ballistic systems and their statistics
can be expressed in terms of the scattering matrix connecting incoming and
outgoing wavefunctions. Here we calculate the dependence of correlation
functions of arbitrarily many pairs of scattering matrices at different
energies on the Ehrenfest time using trajectory based semiclassical methods.
This enables us to verify the prediction from effective random matrix theory
that one part of the correlation function obtains an exponential damping
depending on the Ehrenfest time, while also allowing us to obtain the
additional contribution which arises from bands of always correlated
trajectories. The resulting Ehrenfest-time dependence, responsible e.g. for
secondary gaps in the density of states of Andreev billiards, can also be seen
to have strong effects on other transport quantities like the distribution of
delay times.Comment: Refereed version. 15 pages, 14 figure
The density of states of chaotic Andreev billiards
Quantum cavities or dots have markedly different properties depending on
whether their classical counterparts are chaotic or not. Connecting a
superconductor to such a cavity leads to notable proximity effects,
particularly the appearance, predicted by random matrix theory, of a hard gap
in the excitation spectrum of quantum chaotic systems. Andreev billiards are
interesting examples of such structures built with superconductors connected to
a ballistic normal metal billiard since each time an electron hits the
superconducting part it is retroreflected as a hole (and vice-versa). Using a
semiclassical framework for systems with chaotic dynamics, we show how this
reflection, along with the interference due to subtle correlations between the
classical paths of electrons and holes inside the system, are ultimately
responsible for the gap formation. The treatment can be extended to include the
effects of a symmetry breaking magnetic field in the normal part of the
billiard or an Andreev billiard connected to two phase shifted superconductors.
Therefore we are able to see how these effects can remold and eventually
suppress the gap. Furthermore the semiclassical framework is able to cover the
effect of a finite Ehrenfest time which also causes the gap to shrink. However
for intermediate values this leads to the appearance of a second hard gap - a
clear signature of the Ehrenfest time.Comment: Refereed version. 23 pages, 19 figure
Conductance fluctuations in chaotic systems with tunnel barriers
Quantum effects are expected to disappear in the short-wavelength,
semiclassical limit. As a matter of fact, recent investigations of transport
through quantum chaotic systems have demonstrated the exponential suppression
of the weak localization corrections to the conductance and of the Fano factor
for shot-noise when the Ehrenfest time exceeds the electronic dwell time. On
the other hand, conductance fluctuations, an effect of quantum coherence,
retain their universal value in the limit of the ratio of Ehrenfest time over
dwell time to infinity, when the system is ideally coupled to external leads.
Motivated by this intriguing result we investigate conductance fluctuations
through quantum chaotic cavities coupled to external leads via (tunnel)
barriers of arbitrary transparency. Using the trajectory-based semiclassical
theory of transport, we find a linear Ehrenfest time-dependence of the
conductance variance showing a nonmonotonous, sinusoidal behavior as a function
of the transperancy. Most notably, we find an increase of the conductance
fluctuations with the Ehrenfest time, above their universal value, for the
transparency less than 0.5. These results, confirmed by numerical simulations,
show that, contrarily to the common wisdom, effects of quantum coherence may
increase in the semiclassical limit, under special circumstances
The semiclassical origin of curvature effects in universal spectral statistics
We consider the energy averaged two-point correlator of spectral determinants
and calculate contributions beyond the diagonal approximation using
semiclassical methods. Evaluating the contributions originating from
pseudo-orbit correlations in the same way as in [S. Heusler {\textit {et al.}}\
2007 Phys. Rev. Lett. {\textbf{98}}, 044103] we find a discrepancy between the
semiclassical and the random matrix theory result. A complementary analysis
based on a field-theoretical approach shows that the additional terms occurring
in semiclassics are cancelled in field theory by so-called curvature effects.
We give the semiclassical interpretation of the curvature effects in terms of
contributions from multiple transversals of periodic orbits around shorter
periodic orbits and discuss the consistency of our results with previous
approaches
Loschmidt echo for local perturbations: non-monotonous cross-over from the Fermi-golden-rule to the escape-rate regime
We address the sensitivity of quantum mechanical time evolution by
considering the time decay of the Loschmidt echo (LE) (or fidelity) for local
perturbations of the Hamiltonian. Within a semiclassical approach we derive
analytical expressions for the LE decay for chaotic systems for the whole range
from weak to strong local perturbations and identify different decay regimes
which complement those known for the case of global perturbations. For weak
perturbations a Fermi-golden-rule (FGR) type behavior is recovered. For strong
perturbations the escape-rate regime is reached, where the LE decays
exponentially with a rate independent of the perturbation strength. The
transition between the FGR regime and the escape-rate regime is non-monotonic,
i.e. the rate of the exponential time-decay of the LE oscillates as a function
of the perturbation strength. We further perform extensive quantum mechanical
calculations of the LE based on numerical wave packet evolution which strongly
support our semiclassical theory. Finally, we discuss in some detail possible
experimental realizations for observing the predicted behavior of the LE.Comment: 27 pages, 7 figures; important changes throughout the pape
Periodic-orbit theory of universal level correlations in quantum chaos
Using Gutzwiller's semiclassical periodic-orbit theory we demonstrate
universal behaviour of the two-point correlator of the density of levels for
quantum systems whose classical limit is fully chaotic. We go beyond previous
work in establishing the full correlator such that its Fourier transform, the
spectral form factor, is determined for all times, below and above the
Heisenberg time. We cover dynamics with and without time reversal invariance
(from the orthogonal and unitary symmetry classes). A key step in our reasoning
is to sum the periodic-orbit expansion in terms of a matrix integral, like the
one known from the sigma model of random-matrix theory.Comment: 44 pages, 11 figures, changed title; final version published in New
J. Phys. + additional appendices B-F not included in the journal versio
Multiparticle Correlations in Mesoscopic Scattering: Boson Sampling, Birthday Paradox, and Hong-Ou-Mandel Profiles
The semiclassical continuity equation for open chaotic systems
We consider the continuity equation for open chaotic quantum systems in the
semiclassical limit. First we explicitly calculate a semiclassical expansion
for the probability current density using an expression based on classical
trajectories. The current density is related to the survival probability via
the continuity equation, and we show that this relation is satisfied within the
semiclassical approximation to all orders. For this we develop recursion
relation arguments which connect the trajectory structures involved for the
survival probability, which travel from one point in the bulk to another, to
those structures involved for the current density, which travel from the bulk
to the lead. The current density can also be linked, via another continuity
equation, to a correlation function of the scattering matrix whose
semiclassical approximation is expressed in terms of trajectories that start
and end in the lead. We also show that this continuity equation holds to all
orders.Comment: Refereed version: Minor changes in presentation (especially in the
Introduction) and minor corrections to the start of section 6. 21 pages, 1
figure in two part
Does green tea affect postprandial glucose, insulin and satiety in healthy subjects: a randomized controlled trial
<p>Abstract</p> <p>Background</p> <p>Results of epidemiological studies have suggested that consumption of green tea could lower the risk of type 2 diabetes. Intervention studies show that green tea may decrease blood glucose levels, and also increase satiety. This study was conducted to examine the postprandial effects of green tea on glucose levels, glycemic index, insulin levels and satiety in healthy individuals after the consumption of a meal including green tea.</p> <p>Methods</p> <p>The study was conducted on 14 healthy volunteers, with a crossover design. Participants were randomized to either 300 ml of green tea or water. This was consumed together with a breakfast consisting of white bread and sliced turkey. Blood samples were drawn at 0, 15, 30, 45, 60, 90, and 120 minutes. Participants completed several different satiety score scales at the same times.</p> <p>Results</p> <p>Plasma glucose levels were higher 120 min after ingestion of the meal with green tea than after the ingestion of the meal with water. No significant differences were found in serum insulin levels, or the area under the curve for glucose or insulin. Subjects reported significantly higher satiety, having a less strong desire to eat their favorite food and finding it less pleasant to eat another mouthful of the same food after drinking green tea compared to water.</p> <p>Conclusions</p> <p>Green tea showed no glucose or insulin-lowering effect. However, increased satiety and fullness were reported by the participants after the consumption of green tea.</p> <p>Trial registration number</p> <p>NCT01086189</p
- …