340 research outputs found
Scanning gate experiments: from strongly to weakly invasive probes
An open resonator fabricated in a two-dimensional electron gas is used to
explore the transition from strongly invasive scanning gate microscopy to the
perturbative regime of weak tip-induced potentials. With the help of numerical
simulations that faithfully reproduce the main experimental findings, we
quantify the extent of the perturbative regime in which the tip-induced
conductance change is unambiguously determined by properties of the unperturbed
system. The correspondence between the experimental and numerical results is
established by analyzing the characteristic length scale and the amplitude
modulation of the conductance change. In the perturbative regime, the former is
shown to assume a disorder-dependent maximum value, while the latter linearly
increases with the strength of a weak tip potential.Comment: 11 pages, 7 figure
Shot noise from action correlations
We consider universal shot noise in ballistic chaotic cavities from a
semiclassical point of view and show that it is due to action correlations
within certain groups of classical trajectories. Using quantum graphs as a
model system we sum these trajectories analytically and find agreement with
random-matrix theory. Unlike all action correlations which have been considered
before, the correlations relevant for shot noise involve four trajectories and
do not depend on the presence of any symmetry.Comment: 4 pages, 2 figures (a mistake in version 1 has been corrected
Growth and optical properties of GaN/AlN quantum wells
We demonstrate the growth of GaN/AlN quantum well structures by
plasma-assisted molecular-beam epitaxy by taking advantage of the surfactant
effect of Ga. The GaN/AlN quantum wells show photoluminescence emission with
photon energies in the range between 4.2 and 2.3 eV for well widths between 0.7
and 2.6 nm, respectively. An internal electric field strength of
MV/cm is deduced from the dependence of the emission energy on the well width.Comment: Submitted to AP
Electronic lifetimes in ballistic quantum dots electrostatically coupled to metallic environments
We calculate the lifetime of low-energy electronic excitations in a
two-dimensional quantum dot near a metallic gate. We find different behaviors
depending on the relative values of the dot size, the dot-gate distance and the
Thomas-Fermi screening length within the dot. The standard Fermi liquid
behavior is obtained when the dot-gate distance is much shorter than the dot
size or when it is so large that intrinsic effects dominate. Departures from
the Fermi liquid behavior are found in the unscreened dipole case of small dots
far away from the gate, for which a Caldeira-Leggett model is applicable. At
intermediate distances, a marginal Fermi liquid is obtained if there is
sufficient screening within the dot. In these last two non-trivial cases, the
level width decays as a power law with the dot-gate distance
On general relation between quantum ergodicity and fidelity of quantum dynamics
General relation is derived which expresses the fidelity of quantum dynamics,
measuring the stability of time evolution to small static variation in the
hamiltonian, in terms of ergodicity of an observable generating the
perturbation as defined by its time correlation function. Fidelity for ergodic
dynamics is predicted to decay exponentially on time-scale proportional to
delta^(-2) where delta is the strength of perturbation, whereas faster,
typically gaussian decay on shorter time scale proportional to delta^(-1) is
predicted for integrable, or generally non-ergodic dynamics. This surprising
result is demonstrated in quantum Ising spin-1/2 chain periodically kicked with
a tilted magnetic field where we find finite parameter-space regions of
non-ergodic and non-integrable motion in thermodynamic limit.Comment: Slightly revised version, 4.5 RevTeX pages, 2 figure
Recurrence of fidelity in near integrable systems
Within the framework of simple perturbation theory, recurrence time of
quantum fidelity is related to the period of the classical motion. This
indicates the possibility of recurrence in near integrable systems. We have
studied such possibility in detail with the kicked rotor as an example. In
accordance with the correspondence principle, recurrence is observed when the
underlying classical dynamics is well approximated by the harmonic oscillator.
Quantum revivals of fidelity is noted in the interior of resonances, while
classical-quantum correspondence of fidelity is seen to be very short for
states initially in the rotational KAM region.Comment: 13 pages, 6 figure
Estimating purity in terms of correlation functions
We prove a rigorous inequality estimating the purity of a reduced density
matrix of a composite quantum system in terms of cross-correlation of the same
state and an arbitrary product state. Various immediate applications of our
result are proposed, in particular concerning Gaussian wave-packet propagation
under classically regular dynamics.Comment: 3 page
Asymmetry of tensile vs. compressive elasticity and permeability contributes to the regulation of exchanges in collagen gels
The Starling principle describes exchanges in tissues based on the balance of
hydrostatic and osmotic flows. This balance neglects the coupling between
mechanics and hydrodynamics, a questionable assumption in strained elastic
tissues due to intravascular pressure. Here, we measure the elasticity and
permeability of collagen gels under tensile and compressive stress via the
comparison of the temporal evolution of pressure in an air cavity sealed at the
outlet of a collagen slab with an analytical kinetic model. We observe a drop
in the permeability and enhanced strain-stiffening of native collagen gels
under compression, both effects being essentially lost after chemical
cross-linking. Further, we prove that this asymmetric response accounts for the
accumulation of compressive stress upon sinusoidal fluid injection, which
modulates the material's permeability. Our results thus show that the
properties of collagen gels regulate molecular exchanges and could help
understand drug transport in tissues
Evolution of entanglement under echo dynamics
Echo dynamics and fidelity are often used to discuss stability in quantum
information processing and quantum chaos. Yet fidelity yields no information
about entanglement, the characteristic property of quantum mechanics. We study
the evolution of entanglement in echo dynamics. We find qualitatively different
behavior between integrable and chaotic systems on one hand and between random
and coherent initial states for integrable systems on the other. For the latter
the evolution of entanglement is given by a classical time scale. Analytic
results are illustrated numerically in a Jaynes Cummings model.Comment: 5 RevTeX pages, 3 EPS figures (one color) ; v2: considerable revision
;inequality proof omitte
Signatures of Inelastic Scattering in Coulomb-Blockade Quantum Dots
We calculate the finite-temperature conductance peak-height distributions in
Coublomb-blockade quantum dots in the limit where the inelastic scattering rate
in the dot is large compared with the mean elastic tunneling rate. The relative
reduction of the standard deviation of the peak-height distribution by a
time-reversal symmetry-breaking magnetic field, which is essentially
temperature-independent in the elastic limit, is enhanced by the inclusion of
inelastic scattering at finite temperature. We suggest this quantity as an
independent experimental probe for inelastic scattering in closed dots.Comment: 4 pages, 3 eps figures, revtex
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