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 $\sigma$ bands near the $\bar{\Gamma}$-point. For larger binding energies of these bands, as well as for the $\pi$ band, we find evidence for a substantial lifetime broadening from interband scattering $\pi \rightarrow \sigma$ and $\sigma \rightarrow \pi$, respectively, driven by the out-of-plane ZA acoustic phonons. The essential features of the calculated $\sigma$ band linewidths are in agreement with recent published ARPES data [F. Mazzola et al., Phys.~Rev.~B. 95, 075430 (2017)] and of the $\pi$ 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