13,509 research outputs found
Nonlinear effects of phonon fluctuations on transport through nanoscale junctions
We analyze the effect of electron-phonon coupling on the full counting
statistics of a molecular junction beyond the lowest order perturbation theory.
Our approach allows to take into account analytically the feedback between the
non-equilibrium phonon and electronic distributions in the quantum regime. We
show that even for junctions with high transmission and relatively weak
electron-phonon coupling this feedback gives rise to increasingly higher
nonlinearities in the voltage dependence of the cumulants of the transmitted
charges distribution.Comment: 4 pages, 3 figure
Two-fluid model for a rotating trapped Fermi gas in the BCS phase
We investigate the dynamical properties of a superfluid gas of trapped
fermionic atoms in the BCS phase. As a simple example we consider the reaction
of the gas to a slow rotation of the trap. It is shown that the currents
generated by the rotation can be understood within a two-fluid model similar to
the one used in the theory of superconductors, but with a position dependent
ratio of normal and superfluid densities. The rather general result of this
paper is that already at very low temperatures, far below the critical one, an
important normal-fluid component appears in the outer regions of the gas. This
renders the experimental observation of superfluidity effects more difficult
and indicates that reliable theoretical predictions concerning other dynamical
properties, like the frequencies of collective modes, can only be made by
taking into account temperature effects.Comment: 6 pages, 4 figure
Lobster Eye X-ray optics for astrophysics: Recent status
X-ray optics in Lobster Eye arrangement represent promising complementary device to narrow field X-ray optics in common use. We present briefly recent status of design, developments, and tests of X-ray optics including Lobster Eye modules developed and tested within recent space project
Premise Selection for Mathematics by Corpus Analysis and Kernel Methods
Smart premise selection is essential when using automated reasoning as a tool
for large-theory formal proof development. A good method for premise selection
in complex mathematical libraries is the application of machine learning to
large corpora of proofs. This work develops learning-based premise selection in
two ways. First, a newly available minimal dependency analysis of existing
high-level formal mathematical proofs is used to build a large knowledge base
of proof dependencies, providing precise data for ATP-based re-verification and
for training premise selection algorithms. Second, a new machine learning
algorithm for premise selection based on kernel methods is proposed and
implemented. To evaluate the impact of both techniques, a benchmark consisting
of 2078 large-theory mathematical problems is constructed,extending the older
MPTP Challenge benchmark. The combined effect of the techniques results in a
50% improvement on the benchmark over the Vampire/SInE state-of-the-art system
for automated reasoning in large theories.Comment: 26 page
Natural extension of the Generalised Uncertainty Principle
We discuss a gedanken experiment for the simultaneous measurement of the
position and momentum of a particle in de Sitter spacetime. We propose an
extension of the so-called generalized uncertainty principle (GUP) which
implies the existence of a minimum observable momentum. The new GUP is directly
connected to the nonzero cosmological constant, which becomes a necessary
ingredient for a more complete picture of the quantum spacetime.Comment: 4 pages, 1 figure, v2 with added references, revised and extended as
published in CQ
Large-Scale Magnetic Fields, Dark Energy and QCD
Cosmological magnetic fields are being observed with ever increasing
correlation lengths, possibly reaching the size of superclusters, therefore
disfavouring the conventional picture of generation through primordial seeds
later amplified by galaxy-bound dynamo mechanisms. In this paper we put forward
a fundamentally different approach that links such large-scale magnetic fields
to the cosmological vacuum energy. In our scenario the dark energy is due to
the Veneziano ghost (which solves the problem in QCD). The Veneziano
ghost couples through the triangle anomaly to the electromagnetic field with a
constant which is unambiguously fixed in the standard model. While this
interaction does not produce any physical effects in Minkowski space, it
triggers the generation of a magnetic field in an expanding universe at every
epoch. The induced energy of the magnetic field is thus proportional to
cosmological vacuum energy: , hence acting as a source for the magnetic energy
. The corresponding numerical estimate leads to a magnitude in the
nG range. There are two unique and distinctive predictions of our proposal: an
uninterrupted active generation of Hubble size correlated magnetic fields
throughout the evolution of the universe; the presence of parity violation on
the enormous scales , which apparently has been already observed in CMB.
These predictions are entirely rooted into the standard model of particle
physics.Comment: jhep style, 22 pages, v2 with updated estimates and extended
discussion on parity violation, v3 as published (references updated
Perturbation of magnetostatic modes observed by ferromagnetic resonance force microscopy
Magnetostatic modes of yttrium iron garnet (YIG) films are investigated by ferromagnetic resonance force microscopy. A thin-film "probe" magnet at the tip of a compliant cantilever introduces a local inhomogeneity in the internal field of the YIG sample. This influences the shape of the sample's magnetostatic modes, thereby measurably perturbing the strength of the force coupled to the cantilever. We present a theoretical model that explains these observations; it shows that the tip-induced variation of the internal field creates either a local "potential barrier" or "potential well" for the magnetostatic waves. The data and model together indicate that local magnetic imaging of ferromagnets is possible, even in the presence of long-range spin coupling, through the introduction of localized magnetostatic modes predicted to arise from sufficiently strong tip fields
In Situ ATR-SEIRAS of Carbon Dioxide Reduction at a Plasmonic Silver Cathode.
Illumination of a voltage-biased plasmonic Ag cathode during CO2 reduction results in a suppression of the H2 evolution reaction while enhancing CO2 reduction. This effect has been shown to be photonic rather than thermal, but the exact plasmonic mechanism is unknown. Here, we conduct an in situ ATR-SEIRAS (attenuated total reflectance-surface-enhanced infrared absorption spectroscopy) study of a sputtered thin film Ag cathode on a Ge ATR crystal in CO2-saturated 0.1 M KHCO3 over a range of potentials under both dark and illuminated (365 nm, 125 mW cm-2) conditions to elucidate the nature of this plasmonic enhancement. We find that the onset potential of CO2 reduction to adsorbed CO on the Ag surface is -0.25 VRHE and is identical in the light and the dark. As the production of gaseous CO is detected in the light near this onset potential but is not observed in the dark until -0.5 VRHE, we conclude that the light must be assisting the desorption of CO from the surface. Furthermore, the HCO3- wavenumber and peak area increase immediately upon illumination, precluding a thermal effect. We propose that the enhanced local electric field that results from the localized surface plasmon resonance (LSPR) is strengthening the HCO3- bond, further increasing the local pH. This would account for the decrease in H2 formation and increase the CO2 reduction products in the light
Gravitational particle production in braneworld cosmology
Gravitational particle production in time variable metric of an expanding
universe is efficient only when the Hubble parameter is not too small in
comparison with the particle mass. In standard cosmology, the huge value of the
Planck mass makes the mechanism phenomenologically irrelevant. On the
other hand, in braneworld cosmology the expansion rate of the early universe
can be much faster and many weakly interacting particles can be abundantly
created. Cosmological implications are discussed.Comment: 4 pages, 1 figure, v3 with new definition of and minor text
modification
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