6,860 research outputs found
Quantum Interference and Decoherence in Single-Molecule Junctions: How Vibrations Induce Electrical Current
Quantum interference effects and decoherence mechanisms in single-molecule
junctions are analyzed employing a nonequilibrium Green's function approach.
Electrons tunneling through quasi-degenerate states of a nanoscale molecular
junction exhibit interference effects. We show that electronic-vibrational
coupling, inherent to any molecular junction, strongly quenches such
interference effects. As a result, the electrical current can be significantly
larger than without electronic-vibrational coupling. The analysis reveals that
the quenching of quantum interference is particularly pronounced if the
junction is vibrationally highly excited, e.g. due to current-induced
nonequilibrium effects in the resonant transport regime.Comment: 11 pages, 4 figure
Localized to extended states transition for two interacting particles in a two-dimensional random potential
We show by a numerical procedure that a short-range interaction induces
extended two-particle states in a two-dimensional random potential. Our
procedure treats the interaction as a perturbation and solve Dyson's equation
exactly in the subspace of doubly occupied sites. We consider long bars of
several widths and extract the macroscopic localization and correlation lengths
by an scaling analysis of the renormalized decay length of the bars. For ,
the critical disorder found is , and the critical
exponent . For two non-interacting particles we do not find any
transition and the localization length is roughly half the one-particle value,
as expected.Comment: 4 two-column pages, 4 eps figures, Revtex, to be published in
Europhys. Let
Rolling-sliding laboratory tests of friction modifiers in dry and wet wheel-rail contacts
Friction management has been carried out extensively in the majority of railway networks in the last few years. A popular practice is the application of friction modifiers to increase the adhesion level in contaminated wheel-rail contacts. Two friction modifiers have particularly been used or tested on several railway networks as adhesion enhancers to facilitate the traction and braking operation under poor adhesion conditions. However, for assessment of the performance the railway operators and infrastructure managers mostly rely on practical observations that do not elucidate completely the effectiveness and side effects of these adhesion enhancers. In this paper, the constituents of the two friction modifiers are identified and the solid components are analyzed. A twin-disk roller rig has been used to study their performance in dry and wet contacts under closely controlled laboratory conditions. The adhesion characteristics of both friction modifiers are examined for different slip ratios. Furthermore, the wheel and rail disks are examined after a series of dry tests to analyze the mass loss, the surface damage, the change in surface hardness and roughness, and the subsurface deformation caused by the friction modifiers compared to dry clean contacts. (C) 2009 Elsevier B.V. All rights reserved
Sensitivity study of surface wind flow of a limited area model simulating the extratropical storm Delta affecting the Canary Islands
In November 2005 an extratropical storm named Delta affected the Canary Islands (Spain). The high sustained wind and intense gusts experienced caused significant damage. A numerical sensitivity study of Delta was conducted using the Weather Research & Forecasting Model (WRF-ARW). A total of 27 simulations were performed. Non-hydrostatic and hydrostatic experiments were designed taking into account physical parameterizations and geometrical factors (size and position of the outer domain, definition or not of nested grids, horizontal resolution and number of vertical levels). The Factor Separation Method was applied in order to identify the major model sensitivity parameters under this unusual meteorological situation. Results associated to percentage changes relatives to a control run simulation demonstrated that boundary layer and surface layer schemes, horizontal resolutions, hydrostaticity option and nesting grid activation were the model configuration parameters with the greatest impact on the 48 h maximum 10 m horizontal wind speed solution
Supercurrent and Andreev bound state dynamics in superconducting quantum point contacts under microwave irradiation
We present here an extensive theoretical analysis of the supercurrent of a
superconducting point contact of arbitrary transparency in the presence of a
microwave field. Our study is mainly based on two different approaches: a
two-level model that describes the dynamics of the Andreev bound states in
these systems and a fully microscopic method based on the Keldysh-Green
function technique. This combination provides both a deep insight into the
physics of irradiated Josephson junctions and quantitative predictions for
arbitrary range of parameters. The main predictions of our analysis are: (i)
for weak fields and low temperatures, the microwaves can induce transitions
between the Andreev states leading to a large suppression of the supercurrent
at certain values of the phase, (ii) at strong fields, the current-phase
relation is strongly distorted and the corresponding critical current does not
follow a simple Bessel-function-like behavior, and (iii) at finite temperature,
the microwave field can enhance the critical current by means of transitions
connecting the continuum of states outside the gap region and the Andreev
states inside the gap. Our study is of relevance for a large variety of
superconducting weak links as well as for the proposals of using the Andreev
bound states of a point contact for quantum computing applications.Comment: 16 pages, 11 figures, submitted to Phys. Rev.
Microcavities coupled to multilevel atoms
A three-level atom in the -configuration coupled to a microcavity is
studied. The two transitions of the atom are assumed couple to different
counterpropagating mode pairs in the cavity. We analyze the dynamics both, in
the strong-coupling and the bad cavity limit. We find that compared to a
two-level setup, the third atomic state and the additional control field modes
crucially modify the system dynamics and enable more advanced control schemes.
All results are explained using appropriate dressed state and eigenmode
representations. As potential applications, we discuss optical switching and
turnstile operations and detection of particles close to the resonator surface.Comment: 14 pages, 9 figure
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