118 research outputs found
Electrically driven convection in a thin annular film undergoing circular Couette flow
We investigate the linear stability of a thin, suspended, annular film of
conducting fluid with a voltage difference applied between its inner and outer
edges. For a sufficiently large voltage, such a film is unstable to
radially-driven electroconvection due to charges which develop on its free
surfaces. The film can also be subjected to a Couette shear by rotating its
inner edge. This combination is experimentally realized using films of smectic
A liquid crystals. In the absence of shear, the convective flow consists of a
stationary, azimuthally one-dimensional pattern of symmetric, counter-rotating
vortex pairs. When Couette flow is applied, an azimuthally traveling pattern
results. When viewed in a co-rotating frame, the traveling pattern consists of
pairs of asymmetric vortices. We calculate the neutral stability boundary for
arbitrary radius ratio and Reynolds number of the shear
flow, and obtain the critical control parameter and the critical azimuthal mode number . The
Couette flow suppresses the onset of electroconvection, so that . The calculated suppression is
compared with experiments performed at and .Comment: 17 pages, 2 column with 9 included eps figures. See also
http://mobydick.physics.utoronto.c
Microscopic Theory of Josephson Mesoscopic Constrictions
We present a microscopic theory for the d.c. Josephson effect in model
mesoscopic constrictions. Our method is based on a non-equilibrium Green
function formalism which allows for a self-consistent determination of the
order parameter profile along the constriction. The various regimes defined by
the different length scales (Fermi wavelength , coherence length
and constriction length ) can be analyzed, including the case
where all these lengths are comparable. For the case phase oscillations with spatial period can be
observed. In the case of solutions with a phase-slip center inside
the constriction can be found, in agreement with previous phenomenological
theories.Comment: 4 pages (RevTex 3.0), 3 postscript figures available upon request,
312456-C
Localization and Capacitance Fluctuations in Disordered Au Nano-junctions
Nano-junctions, containing atomic-scale gold contacts between strongly
disordered leads, exhibit different transport properties at room temperature
and at low temperature. At room temperature, the nano-junctions exhibit
conductance quantization effects. At low temperatures, the contacts exhibit
Coulomb-Blockade. We show that the differences between the room-temperature and
low temperature properties arise from the localization of electronic states in
the leads. The charging energy and capacitance of the nano-junctions exhibit
strong fluctuations with applied magnetic field at low temperature, as
predicted theoretically.Comment: 20 pages 8 figure
Quantum interference structures in the conductance plateaus of gold nanojunctions
The conductance of breaking metallic nanojunctions shows plateaus alternated
with sudden jumps, corresponding to the stretching of stable atomic
configurations and atomic rearrangements, respectively. We investigate the
structure of the conductance plateaus both by measuring the voltage dependence
of the plateaus' slope on individual junctions and by a detailed statistical
analysis on a large amount of contacts. Though the atomic discreteness of the
junction plays a fundamental role in the evolution of the conductance, we find
that the fine structure of the conductance plateaus is determined by quantum
interference phenomenon to a great extent.Comment: 4 pages, 4 figure
Free-electron Model for Mesoscopic Force Fluctuations in Nanowires
When two metal electrodes are separated, a nanometer sized wire (nanowire) is
formed just before the contact breaks. The electrical conduction measured
during this retraction process shows signs of quantized conductance in units of
G_0=2e^2/h. Recent experiments show that the force acting on the wire during
separation fluctuates, which has been interpreted as being due to atomic
rearrangements. In this report we use a simple free electron model, for two
simple geometries, and show that the electronic contribution to the force
fluctuations is comparable to the experimentally found values, about 2 nN.Comment: 4 pages, 3 figures, reference correcte
Effect of quantum interference in the nonlinear conductance of microconstrictions
The influence of the interference of electron waves, which were scattered by
single impurities, on nonlinear quantum conductance of metallic
microconstrictions (as was recently investigated experimentally) is studied
theoretically. The dependence of the interference pattern in the conductance
on the contact diameter and the spatial distribution of impurities is
analyzed. It is shown that the amplitude of conductance oscillation is strongly
depended on the position of impurities inside the constriction.Comment: 6 pages, 4 figures, To appear in PR
Electronic transport and vibrational modes in the smallest molecular bridge: H2 in Pt nanocontacts
We present a state-of-the-art first-principles analysis of electronic
transport in a Pt nanocontact in the presence of H2 which has been recently
reported by Smit et al. in Nature 419, 906 (2002). Our results indicate that at
the last stages of the breaking of the Pt nanocontact two basic forms of bridge
involving H can appear. Our claim is, in contrast to Smit et al.'s, that the
main conductance histogram peak at G approx 2e^2/h is not due to molecular H2,
but to a complex Pt2H2 where the H2 molecule dissociates. A first-principles
vibrational analysis that compares favorably with the experimental one also
supports our claim .Comment: 5 pages, 3 figure
Shot noise suppression at room temperature in atomic-scale Au junctions
Shot noise encodes additional information not directly inferable from simple
electronic transport measurements. Previous measurements in atomic-scale metal
junctions at cryogenic temperatures have shown suppression of the shot noise at
particular conductance values. This suppression demonstrates that transport in
these structures proceeds via discrete quantum channels. Using a high frequency
technique, we simultaneously acquire noise data and conductance histograms in
Au junctions at room temperature and ambient conditions. We observe noise
suppression at up to three conductance quanta, with possible indications of
current-induced local heating and noise in the contact region at high
biases. These measurements demonstrate the quantum character of transport at
room temperature at the atomic scale. This technique provides an additional
tool for studying dissipation and correlations in nanodevices.Comment: 15 pages, 4 figures + supporting information (6 pages, 6 figures
Stabilizing single atom contacts by molecular bridge formation
Gold-molecule-gold junctions can be formed by carefully breaking a gold wire
in a solution containing dithiolated molecules. Surprisingly, there is little
understanding on the mechanical details of the bridge formation process and
specifically on the role that the dithiol molecules play themselves. We propose
that alkanedithiol molecules have already formed bridges between the gold
electrodes before the atomic gold-gold junction is broken. This leads to
stabilization of the single atomic gold junction, as observed experimentally.
Our data can be understood within a simple spring model.Comment: 14 pages, 3 figures, 1 tabl
Bifurcations in annular electroconvection with an imposed shear
We report an experimental study of the primary bifurcation in
electrically-driven convection in a freely suspended film. A weakly conducting,
submicron thick smectic liquid crystal film was supported by concentric
circular electrodes. It electroconvected when a sufficiently large voltage
was applied between its inner and outer edges. The film could sustain rapid
flows and yet remain strictly two-dimensional. By rotation of the inner
electrode, a circular Couette shear could be independently imposed. The control
parameters were a dimensionless number , analogous to the Rayleigh
number, which is and the Reynolds number of the
azimuthal shear flow. The geometrical and material properties of the film were
characterized by the radius ratio , and a Prandtl-like number . Using measurements of current-voltage characteristics of a large number of
films, we examined the onset of electroconvection over a broad range of
, and . We compared this data quantitatively to
the results of linear stability theory. This could be done with essentially no
adjustable parameters. The current-voltage data above onset were then used to
infer the amplitude of electroconvection in the weakly nonlinear regime by
fitting them to a steady-state amplitude equation of the Landau form. We show
how the primary bifurcation can be tuned between supercritical and subcritical
by changing and .Comment: 17 pages, 12 figures. Submitted to Phys. Rev. E. Minor changes after
refereeing. See also http://mobydick.physics.utoronto.c
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