1,595 research outputs found
In-situ Particle Acceleration in Collisionless Shocks
The outflows from gamma ray bursts, active galactic nuclei and relativistic
jets in general interact with the surrounding media through collisionless
shocks. With three dimensional relativistic particle-in-cell simulations we
investigate such shocks. The results from these experiments show that
small--scale magnetic filaments with strengths of up to percents of
equipartition are generated and that electrons are accelerated to power law
distributions N(E)~E^{-p} in the vicinity of the filaments through a new
acceleration mechanism. The acceleration is locally confined, instantaneous and
differs from recursive acceleration processes such as Fermi acceleration. We
find that the proposed acceleration mechanism competes with thermalization and
becomes important at high Lorentz factors.Comment: 4 pages, 2 figures, submitted to Il nuovo cimento (4th Workshop
Gamma-Ray Bursts in the Afterglow Era, Rome, 18-22 October 2004
Phonon-induced linewidths of graphene electronic states
The linewidths of the electronic bands originating from the electron-phonon
coupling in graphene are analyzed based on model tight-binding calculations and
experimental angle-resolved photoemission spectroscopy (ARPES) data. Our
calculations confirm the prediction that the high-energy optical phonons
provide the most essential contribution to the phonon-induced linewidth of the
two upper occupied bands near the -point. For larger
binding energies of these bands, as well as for the band, we find
evidence for a substantial lifetime broadening from interband scattering and , respectively, driven by the
out-of-plane ZA acoustic phonons. The essential features of the calculated
band linewidths are in agreement with recent published ARPES data [F.
Mazzola et al., Phys.~Rev.~B. 95, 075430 (2017)] and of the band
linewidth with ARPES data presented here.Comment: 7 pages, 4 figure
Inelastic fingerprints of hydrogen contamination in atomic gold wire systems
We present series of first-principles calculations for both pure and hydrogen
contaminated gold wire systems in order to investigate how such impurities can
be detected. We show how a single H atom or a single H2 molecule in an atomic
gold wire will affect forces and Au-Au atom distances under elongation. We
further determine the corresponding evolution of the low-bias conductance as
well as the inelastic contributions from vibrations. Our results indicate that
the conductance of gold wires is only slightly reduced from the conductance
quantum G0=2e^2/h by the presence of a single hydrogen impurity, hence making
it difficult to use the conductance itself to distinguish between various
configurations. On the other hand, our calculations of the inelastic signals
predict significant differences between pure and hydrogen contaminated wires,
and, importantly, between atomic and molecular forms of the impurity. A
detailed characterization of gold wires with a hydrogen impurity should
therefore be possible from the strain dependence of the inelastic signals in
the conductance.Comment: 5 pages, 3 figures, Contribution to ICN+T2006, Basel, Switzerland,
July-August 200
Non-Fermi Power law Acceleration in Astrophysical Plasma Shocks
Collisionless plasma shock theory, which applies for example to the afterglow
of gamma ray bursts, still contains key issues that are poorly understood. In
this paper we study charged particle dynamics in a highly relativistic
collisionless shock numerically using ~10^9 particles. We find a power law
distribution of accelerated electrons, which upon detailed investigation turns
out to originate from an acceleration mechanism that is decidedly different
from Fermi acceleration.
Electrons are accelerated by strong filamentation instabilities in the
shocked interpenetrating plasmas and coincide spatially with the power law
distributed current filamentary structures. These structures are an inevitable
consequence of the now well established Weibel-like two-stream instability that
operates in relativistic collisionless shocks.
The electrons are accelerated and decelerated instantaneously and locally; a
scenery that differs qualitatively from recursive acceleration mechanisms such
as Fermi acceleration.
The slopes of the electron distribution power laws are in concordance with
the particle power law spectra inferred from observed afterglow synchrotron
radiation in gamma ray bursts, and the mechanism can possibly explain more
generally the origin of non-thermal radiation from shocked inter- and
circum-stellar regions and from relativistic jets.Comment: 4 pages accepted for publication in ApJ Letters. High resolution
figures are available online at http://www.astro.ku.dk/users/hededal/040855
Controlled Contact to a C60 Molecule
The conductance of C60 on Cu(100) is investigated with a low-temperature
scanning tunneling microscope. At the transition from tunneling to the contact
regime the conductance of C60 adsorbed with a pentagon-hexagon bond rises
rapidly to 0.25 conductance quanta G0. An abrupt conductance jump to G0 is
observed upon further decreasing the distance between the instrument's tip and
the surface. Ab-initio calculations within density functional theory and
non-equilibrium Green's function techniques explain the experimental data in
terms of the conductance of an essentially undeformed C60. From a detailed
analysis of the crossover from tunneling to contact we conclude that the
conductance in this region is strongly affected by structural fluctuations
which modulate the tip-molecule distance.Comment: 4 pages, 3 figure
A tractable inhomogeneous closure theory for flow over mean topography
The quasi-diagonal direct interaction approximation (QDIA) is shown to be a computationally tractable closure theory for inhomogeneous two-dimensional turbulent flow over mean (single-realization) topography. In this paper numerical results for the QDIA are compared to direct numerical simulation (DNS) at moderate Reynolds number for two cases with quite different topographic and mean field amplitudes. The QDIA is found to be in excellent agreement with DNS for cases where the small-scale topographic amplitude is significant. For cases where the small-scale topography is weak, the QDIA closely reproduces the evolving mean field and large-scale energy containing transients but under represents the amplitudes of the small-scale transients in a similar way to the homogeneous DIA. We discuss the prospects of ameliorating the small-scale deficiencies using a regularization of the interaction coefficients
Engineering Negative Differential Conductance with the Cu(111) Surface State
Low-temperature scanning tunneling microscopy and spectroscopy are employed
to investigate electron tunneling from a C60-terminated tip into a Cu(111)
surface. Tunneling between a C60 orbital and the Shockley surface states of
copper is shown to produce negative differential conductance (NDC) contrary to
conventional expectations. NDC can be tuned through barrier thickness or C60
orientation up to complete extinction. The orientation dependence of NDC is a
result of a symmetry matching between the molecular tip and the surface states.Comment: 5 pages, 4 figures, 1 tabl
Normal-Superconducting Phase Transition Mimicked by Current Noise
As a superconductor goes from the normal state into the superconducting
state, the voltage vs. current characteristics at low currents change from
linear to non-linear. We show theoretically and experimentally that the
addition of current noise to non-linear voltage vs. current curves will create
ohmic behavior. Ohmic response at low currents for temperatures below the
critical temperature mimics the phase transition and leads to incorrect
values for and the critical exponents and . The ohmic response
occurs at low currents, when the applied current is smaller than the
width of the probability distribution , and will occur in both the
zero-field transition and the vortex-glass transition. Our results indicate
that the transition temperature and critical exponents extracted from the
conventional scaling analysis are inaccurate if current noise is not filtered
out. This is a possible explanation for the wide range of critical exponents
found in the literature.Comment: 4 pages, 2 figure
- …