8,804 research outputs found
Electron transport through a metal-molecule-metal junction
Molecules of bisthiolterthiophene have been adsorbed on the two facing gold
electrodes of a mechanically controllable break junction in order to form
metal-molecule(s)-metal junctions. Current-voltage (I-V) characteristics have
been recorded at room temperature. Zero bias conductances were measured in the
10-100 nS range and different kinds of non-linear I-V curves with step-like
features were reproducibly obtained. Switching between different kinds of I-V
curves could be induced by varying the distance between the two metallic
electrodes. The experimental results are discussed within the framework of
tunneling transport models explicitly taking into account the discrete nature
of the electronic spectrum of the molecule.Comment: 12 pages, 12 figures to appear in Phys. Rev. B 59(19) 199
On Gravitational Radiation in Quadratic Gravity
We investigate the gravitational radiation emitted by an isolated system for
gravity theories with Lagrange density . As a formal result we
obtain leading order corrections to the quadrupole formula in General
Relativity. We make use of the analogy of theories with scalar--tensor
theories, which in contrast to General Relativity feature an additional scalar
degree of freedom. Unlike General Relativity, where the leading order
gravitational radiation is produced by quadrupole moments, the additional
degree of freedom predicts gravitational radiation of all multipoles, in
particular monopoles and dipoles, as this is the case for the most alternative
gravity theories known today. An application to a hypothetical binary pulsar
moving in a circular orbit yields the rough limit by constraining the dipole power to account at
most for 1% of the quadrupole power as predicted by General Relativity.Comment: 14 Pages, 1 Figur
Foraging movements of emperor penguins at Pointe Géologie, Antarctica.
International audienceThe foraging distributions of 20 breeding emperor penguins were investigated at Pointe Ge´ologie, Terre Ade´lie, Antarctica by using satellite telemetry in 2005 and 2006 during early and late winter, as well as during late spring and summer, corresponding to incubation, early chick-brooding, late chick-rearing and the adult pre-moult period, respectively. Dive depth records of three post-egg-laying females, two post-incubating males and four late chick-rearing adults were examined, as well as the horizontal space use by these birds. Foraging ranges of chick-provisioning penguins extended over the Antarctic shelf and were constricted by winter pack-ice. During spring ice break-up, the foraging ranges rarely exceeded the shelf slope, although seawater access was apparently almost unlimited. Winter females appeared constrained in their access to open water but used fissures in the sea ice and expanded their prey search effort by expanding the horizontal search component underwater. Birds in spring however, showed higher area-restricted-search than did birds in winter. Despite different seasonal foraging strategies, chick-rearing penguins exploited similar areas as indicated by both a high ‘Area-Restricted-Search Index' and high ‘Catch Per Unit Effort'. During pre-moult trips, emperor penguins ranged much farther offshore than breeding birds, which argues for particularly profitable oceanic feeding areas which can be exploited when the time constraints imposed by having to return to a central place to provision the chick no longer apply
Classical versus Quantum Time Evolution of Densities at Limited Phase-Space Resolution
We study the interrelations between the classical (Frobenius-Perron) and the
quantum (Husimi) propagator for phase-space (quasi-)probability densities in a
Hamiltonian system displaying a mix of regular and chaotic behavior. We focus
on common resonances of these operators which we determine by blurring
phase-space resolution. We demonstrate that classical and quantum time
evolution look alike if observed with a resolution much coarser than a Planck
cell and explain how this similarity arises for the propagators as well as
their spectra. The indistinguishability of blurred quantum and classical
evolution implies that classical resonances can conveniently be determined from
quantum mechanics and in turn become effective for decay rates of quantum
correlations.Comment: 10 pages, 3 figure
Gravitational lens optical scalars in terms of energy-momentum distributions
This is a general work on gravitational lensing. We present new expressions
for the optical scalars and the deflection angle in terms of the
energy-momentum tensor components of matter distributions. Our work generalizes
standard references in the literature where normally stringent assumptions are
made on the sources. The new expressions are manifestly gauge invariant, since
they are presented in terms of curvature components. We also present a method
of approximation for solving the lens equations, that can be applied to any
order.Comment: 17 pages, 2 figures. Titled changed. Small improvements. References
added. Final version published in Phys.Rev.
First-principles study of electron transport through cages
Electron transport properties of C molecules suspended between gold
electrodes are investigated using first-principles calculations. Our study
reveals that the conductances are quite sensitive to the number of C
molecules between electrodes: the conductances of C monomers are near 1
G, while those of dimers are markedly smaller, since incident electrons
easily pass the C molecules and are predominantly scattered at the
C-C junctions. Moreover, we find both channel currents locally
circulating the outermost carbon atoms.Comment: 8 pages and 3 figure
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