2,176 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
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
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
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
Supervisors and Performance Management Systems
Supervisors occupy central roles in production and performance monitoring. We study how heterogeneity in performance evaluations across supervisors affects employee and supervisor careers and firm outcomes using data on the performance system of a Scandinavian service sector firm. We show that supervisors vary widely in how they rate subordinates of similar quality. To understand the nature of this heterogeneity, we propose a principal-agent model according to which supervisors can differ in their ability to elicit output from subordinates or in their taste for leniency when rating subordinates. The model also allows for variation in how informed firms are about this heterogeneity. Within the context of this model, we can discern the nature of the heterogeneity across supervisors and how informed firms are about this heterogeneity by relating observed supervisor heterogeneity in ratings to worker, supervisor, and firm outcomes. We find that subordinates are paid significantly more, and their pay is more closely aligned with performance, when they are matched to a highrating supervisor. We also find that higher raters themselves are paid more and that the teams managed by higher raters perform better on objective performance measures. This evidence suggests that supervisor heterogeneity stems, at least in part, from real differences in managerial ability and that firms are at least partially informed about these differences. We conclude by quantifying how important heterogeneity in supervisor type is for workers' careers. For a typical worker, matching to a high rater (90th percentile) relative to a low rater (10th percentile) for just one year results in an increase in the present discounted value of earnings equivalent to 7 14% of an annual salary
Your supervisor’s personality impacts you forever
Supervisors are different in their managerial abilities and in how they perceive your work, yet their decisions determine you career outcomes, write Anders Frederiksen, Lisa Kahn, and Fabian Lang
Magnetic Field Generation in Collisionless Shocks; Pattern Growth and Transport
We present results from three-dimensional particle simulations of
collisionless shocks with relativistic counter-streaming ion-electron plasmas.
Particles are followed over many skin depths downstream of the shock. Open
boundaries allow the experiments to be continued for several particle crossing
times. The experiments confirm the generation of strong magnetic and electric
fields by a Weibel-like kinetic streaming instability, and demonstrate that the
electromagnetic fields propagate far downstream of the shock. The magnetic
fields are predominantly transversal, and are associated with merging ion
current channels. The total magnetic energy grows as the ion channels merge,
and as the magnetic field patterns propagate down stream. The electron
populations are quickly thermalized, while the ion populations retain distinct
bulk speeds in shielded ion channels and thermalize much more slowly. These
results may help explain the origin of the magnetic fields responsible for
afterglow synchrotron/jitter radiation from Gamma-Ray Bursts.Comment: 4 pages, 6 figures - Accepted to ApJL. Revised version following
recommendations of referee report. Content reduced marginally. Conclusions
unchange
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