72,786 research outputs found
Collective excitations of Dirac electrons in a graphene layer with spin-orbit interaction
Coulomb screening and excitation spectra of electrons in a graphene layer
with spin-orbit interaction (SOI) is studied in the random phase approximation.
The SOI opens a gap between the valence and conduction bands of the semi-metal
Dirac system and reshapes the bottom and top of the bands. As a result, we have
observed a dramatic change in the long-wavelength dielectric function of the
system. An undamped plasmon mode emerges from the inter-band electron-hole
excitation continuum edge and vanishes or merges with a Landau damped mode on
the edge of the intra-band electron-hole continuum. The characteristic
collective excitation of the Dirac gas is recovered in a system with high
carrier density or for a large wave vector
Liquid-Crystal Blazed-Grating Beam Deflector
A transmission-type nonmechanical multiple-angle beam-steering device that uses liquid-crystal blazed grating has been developed. Sixteen steering angles with a contrast ratio of 18 has been demonstrated. A detailed analysis of the liquid-crystal and poly(methyl methacrylate) blazed-grating deflector was carried out to provide guidance during the deflector’s development. A manufacturing offset compensation technique is proposed to improve the device’s performance greatly. A hybrid approach utilizing electrically generated blazed grating combined with the cascading approach described here yields in excess of 500 deflecting angles
Dynamic modeling of spacecraft in a collisionless plasma
A new computational model is described which can simulate the charging of complex geometrical objects in three dimensions. Two sample calculations are presented. In the first problem, the capacitance to infinity of a complex object similar to a satellite with solar array paddles is calculated. The second problem concerns the dynamical charging of a conducting cube partially covered with a thin dielectric film. In this calculation, the photoemission results in differential charging of the object
Social Turing Tests: Crowdsourcing Sybil Detection
As popular tools for spreading spam and malware, Sybils (or fake accounts)
pose a serious threat to online communities such as Online Social Networks
(OSNs). Today, sophisticated attackers are creating realistic Sybils that
effectively befriend legitimate users, rendering most automated Sybil detection
techniques ineffective. In this paper, we explore the feasibility of a
crowdsourced Sybil detection system for OSNs. We conduct a large user study on
the ability of humans to detect today's Sybil accounts, using a large corpus of
ground-truth Sybil accounts from the Facebook and Renren networks. We analyze
detection accuracy by both "experts" and "turkers" under a variety of
conditions, and find that while turkers vary significantly in their
effectiveness, experts consistently produce near-optimal results. We use these
results to drive the design of a multi-tier crowdsourcing Sybil detection
system. Using our user study data, we show that this system is scalable, and
can be highly effective either as a standalone system or as a complementary
technique to current tools
A Relation between the Anomalous Dimensions and OPE Coefficients in Asymptotic Free Field Theories
In asymptotic free field theories we show that part of the OPE of the trace
of the stress-energy tensor and an arbitrary composite field is determined by
the anomalous dimension of the composite field. We take examples from the
two-dimensional O(N) non-linear sigma model.Comment: 6 pages, plain TeX, uses harvma
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