Non-linear screening of external charge by doped graphene

We solve a nonlinear integral equation for the electrostatic potential in doped graphene due to an external charge, arising from a Thomas-Fermi (TF) model for screening by graphene's $\pi$ electron bands. In particular, we study the effects of a finite equilibrium charge carrier density in graphene, non-zero temperature, non-zero gap between graphene and a dielectric substrate, as well as the nonlinearity in the band density of states. Effects of the exchange and correlation interactions are also briefly discussed for undoped graphene at zero temperature. Nonlinear results are compared with both the linearized TF model and the dielectric screening model within random phase approximation (RPA). In addition, image potential of the external charge is evaluated from the solution of the nonlinear integral equation and compared to the results of linear models. We have found generally good agreement between the results of the nonlinear TF model and the RPA model in doped graphene, apart from Friedel oscillations in the latter model. However, relatively strong nonlinear effects are found in the TF model to persist even at high doping densities and large distances of the external charge.Comment: 12 pages including 6 figure

Correlation-Based Tuning of a Restricted-Complexity Controller for an Active Suspension System

A correlation-based controller tuning method is proposed for the ``Design and optimization of restricted-complexity controllers'' benchmark problem. The approach originally proposed for model following is extended to solve the disturbance rejection problem. The idea is to tune the controller parameters such that the closed-loop output be uncorrelated with the disturbance signal. Since perfect decorrelation between the closed-loop output and the disturbance signal is not attainable in the restricted-complexity controller design, the cross correlation between these two signals is minimized iteratively using the stochastic approximation method. Since control specifications can normally be expressed in terms of constraints on the sensitivity functions, a frequency-domain analysis of the criterion is performed. Straightforward implementation of the proposed approach on the active suspension system of the Automatic Control Laboratory of Grenoble (LAG) provides a 2nd-order controller that meets the control specifications very well

Iterative Correlation-Based Controller Tuning: Application to a Magnetic Suspension System

Iterative tuning of the parameters of a restricted-order controller using the data acquired in closed-loop operation seems to be a promising idea, especially for tuning PID controllers in industrial applications. In this paper, a new tuning approach based on decorrelation is proposed. The basic idea is to make the output error between the designed and achieved closed-loop systems uncorrelated with the reference signal. The controller parameters are calculated as the solution to correlation equations involving instrumental variables. Different choices of instrumental variables are proposed and compared via simulation. The stochastic properties of the correlation approach are compared with those of standard IFT using Monte-Carlo simulation. The proposed approach is also implemented on an experimental magnetic suspension system, and excellent performance using only a few real-time experiments is achieved

Randomized controlled trial investigating the effect of music on the virtual reality laparoscopic learning performance of novice surgeons

Background: Findings have shown that music affects cognitive performance, but little is known about its influence on surgical performance. The hypothesis of this randomized controlled trial was that arousing (activating) music has a beneficial effect on the surgical performance of novice surgeons in the setting of a laparoscopic virtual reality task. Methods: For this study, 45 junior surgeons with no previous laparoscopic experience were randomly assigned to three equal groups. Group 1 listened to activating music; group 2 listened to deactivating music; and group 3 had no music (control) while each participant solved a surgical task five times on a virtual laparoscopic simulator. The assessed global task score, the total task time, the instrument travel distances, and the surgeons' heart rate were assessed. Results: All surgical performance parameters improved significantly with experience (task repetition). The global score showed a trend for a between-groups difference, suggesting that the group listening to activating music had the worst performance. This observation was supported by a significant between-groups difference for the first trial but not subsequent trials (activating music, 35 points; deactivating music, 66 points; no music, 91 points; p=0.002). The global score (p=0.056) and total task time (p=0.065) showed a trend toward improvement when participants considered the music pleasant rather than unpleasant. Conclusions: Music in the operating theater may have a distracting effect on novice surgeons performing new tasks. Surgical trainers should consider categorically switching off music during teaching procedure

N = 2 AdS4 supergravity, holography and Ward identities

We develop in detail the holographic framework for an N = 2 pure AdS supergravity model in four dimensions, including all the contributions from the fermionic fields and adopting the Fefferman-Graham parametrization. We work in the first order formalism, where the full superconformal structure can be kept manifest in principle, even if only a part of it is realized as a symmetry on the boundary, while the remainder has a non-linear realization. Our study generalizes the results presented in antecedent literature and includes a general discussion of the gauge-fixing conditions on the bulk fields which yield the asymptotic symmetries at the boundary. We construct the corresponding super- conformal currents and show that they satisfy the related Ward identities when the bulk equations of motion are imposed. Consistency of the holographic setup requires the super- AdS curvatures to vanish at the boundary. This determines, in particular, the expression of the super-Schouten tensor of the boundary theory, which generalizes the purely bosonic Schouten tensor of standard gravity by including gravitini bilinears. The same applies to the superpartner of the super-Schouten tensor, the conformino. Furthermore, the vanishing of the supertorsion poses general constraints on the sources of the three-dimensional boundary conformal field theory and requires that the super-Schouten tensor is endowed with an antisymmetric part proportional to a gravitino-squared term

Wake effect in graphene due to moving charged particles

We study the wake effect in a supported graphene layer induced by external charged particles moving parallel to it by using the dynamic polarization function of graphene within the random phase approximation for its pi electrons described as Diracs fermions. We explore the effects of a substrate assuming that graphene is supported by an insulating substrate, such as SiO2, and a strongly polar substrate, such as SiC, under the gating conditions. Strong effects are observed in the wake pattern in the induced density of charge carriers in supported graphene due to finite size of the graphene-substrate gap, as well as due to strong coupling effects, and plasmon damping of graphenes pi electrons. We find that the excitation of surface phonons in the substrate may exert quite strong influences on the wake effect in the total electrostatic potential in the graphene plane at low particle speeds.27th Summer School and International Symposium on the Physics of Ionized Gases (SPIG), Aug 26-29, 2014, Serbian Acad Sci and Arts, Belgrade, Serbi

The role of low-level image features in the affective categorization of rapidly presented scenes

It remains unclear how the visual system is able to extract affective content from complex scenes even with extremely brief (\u3c 100 millisecond) exposures. One possibility, suggested by findings in machine vision, is that low-level features such as unlocalized, two-dimensional (2-D) Fourier spectra can be diagnostic of scene content. To determine whether Fourier image amplitude carries any information about the affective quality of scenes, we first validated the existence of image category differences through a support vector machine (SVM) model that was able to discriminate our intact aversive and neutral images with ~ 70% accuracy using amplitude-only features as inputs. This model allowed us to confirm that scenes belonging to different affective categories could be mathematically distinguished on the basis of amplitude spectra alone. The next question is whether these same features are also exploited by the human visual system. Subsequently, we tested observers’ rapid classification of affective and neutral naturalistic scenes, presented briefly (~33.3 ms) and backward masked with synthetic textures. We tested categorization accuracy across three distinct experimental conditions, using: (i) original images, (ii) images having their amplitude spectra swapped within a single affective image category (e.g., an aversive image whose amplitude spectrum has been swapped with another aversive image) or (iii) images having their amplitude spectra swapped between affective categories (e.g., an aversive image containing the amplitude spectrum of a neutral image). Despite its discriminative potential, the human visual system does not seem to use Fourier amplitude differences as the chief strategy for affectively categorizing scenes at a glance. The contribution of image amplitude to affective categorization is largely dependent on interactions with the phase spectrum, although it is impossible to completely rule out a residual role for unlocalized 2-D amplitude measures

Interactions of ions with graphene

We evaluate the stopping and image forces on a charged particle moving parallel to a single sheet of graphene supported by an insulating substrate under the gating conditions. The forces are presented as functions of the particle speed and the particle distance for a broad range of charge-carrier densities in graphene. We also consider the effects of a finite gap between graphene and a supporting substrate, as well as the effects of a finite damping rate that is included through the use of Mermins procedure. The damping rate is estimated from a tentative comparison of the Mermin loss function with a high-resolution reflection electron energy loss spectroscopy experiment.25th Summer School and International Symposium on the Physics of Ionized Gases - SPIG 2010, Aug 30-Sep 03, 2010, Donji Milanovac, Serbi