Quantum Hall effect in exfoliated graphene affected by charged impurities: metrological measurements

Metrological investigations of the quantum Hall effect (QHE) completed by transport measurements at low magnetic field are carried out in a-few-$\mu\mathrm{m}$-wide Hall bars made of monolayer (ML) or bilayer (BL) exfoliated graphene transferred on $\textrm{Si/SiO}_{2}$ substrate. From the charge carrier density dependence of the conductivity and from the measurement of the quantum corrections at low magnetic field, we deduce that transport properties in these devices are mainly governed by the Coulomb interaction of carriers with a large concentration of charged impurities. In the QHE regime, at high magnetic field and low temperature ($T<1.3 \textrm{K}$), the Hall resistance is measured by comparison with a GaAs based quantum resistance standard using a cryogenic current comparator. In the low dissipation limit, it is found quantized within 5 parts in $10^{7}$ (one standard deviation, $1 \sigma$) at the expected rational fractions of the von Klitzing constant, respectively $R_{\mathrm{K}}/2$ and $R_{\mathrm{K}}/4$ in the ML and BL devices. These results constitute the most accurate QHE quantization tests to date in monolayer and bilayer exfoliated graphene. It turns out that a main limitation to the quantization accuracy, which is found well above the $10^{-9}$ accuracy usually achieved in GaAs, is the low value of the QHE breakdown current being no more than $1 \mu\mathrm{A}$. The current dependence of the longitudinal conductivity investigated in the BL Hall bar shows that dissipation occurs through quasi-elastic inter-Landau level scattering, assisted by large local electric fields. We propose that charged impurities are responsible for an enhancement of such inter-Landau level transition rate and cause small breakdown currents.Comment: 14 pages, 9 figure

A positive interaction between inhibitors of protein synthesis and cefepime in the fight against methicillin-resistant Staphylococcus aureus

Quinupristin-dalfopristin (Q-D) synergizes with cefepime for the treatment of methicillin-resistant Staphylococcus aureus (MRSA). Here, we studied whether the synergism was restricted to MRSA and if it extended to non-beta-lactam cell wall inhibitors or to other inhibitors of protein synthesis. Three MRSA and two methicillin-susceptible S. aureus (MSSA) strains were tested, including an isogenic pair of mecA −/mecA + S. aureus Newman. The drug interactions were determined by fractional inhibitory concentration (FIC) indices and population analysis profiles. The antibacterial drugs that we used included beta-lactam (cefepime) and non-beta-lactam cell wall inhibitors (D-cycloserine, fosfomycin, vancomycin, teicoplanin), inhibitors of protein synthesis (Q-D, erythromycin, chloramphenicol, tetracycline, linezolid, fusidic acid), and polynucleotide inhibitors (cotrimoxazole, ciprofloxacin). The addition of each protein inhibitor to cefepime was synergistic (FIC ≤ 0.5) or additive (FIC > 0.5 but < 1) against MRSA, but mostly indifferent against MSSA (FIC ≥ 1 but ≤ 4). This segregation was not observed after adding cotrimoxazole or ciprofloxacin to cefepime. Population analysis profiles were performed on plates in the presence of increasing concentrations of the cell wall inhibitors plus 0.25 × minimum inhibitory concentration (MIC) of Q-D. Cefepime combined with Q-D was synergistic against MRSA, but D-cycloserine and glycopeptides were not. Thus, the synergism was specific to beta-lactam antibiotics. Moreover, the synergism was not lost against fem mutants, indicating that it acted at another level. The restriction of the beneficial effect to MRSA suggests that the functionality of penicillin-binding protein 2A (PBP2A) was affected, either directly or indirectly. Further studies are necessary in order to provide a mechanism for this positive interactio

A 30 GHz 5-TeV Linear Collider

We present parameters for a linear collider with a $3$ to $5$ TeV center-of-mass energy that utilizes conventional rf technology operating at a frequency around 30 GHz. We discuss the scaling laws and assumed limitations that lead to the parameters described and we compare the merits and liabilities of different technological options including rf power source, accelerator structure, and final focus system design. Finally, we outline the components of the collider while specifying the required alignment and construction tolerances

A posteriori error analysis and adaptive non-intrusive numerical schemes for systems of random conservation laws

In this article we consider one-dimensional random systems of hyperbolic conservation laws. We first establish existence and uniqueness of random entropy admissible solutions for initial value problems of conservation laws which involve random initial data and random flux functions. Based on these results we present an a posteriori error analysis for a numerical approximation of the random entropy admissible solution. For the stochastic discretization, we consider a non-intrusive approach, the Stochastic Collocation method. The spatio-temporal discretization relies on the Runge--Kutta Discontinuous Galerkin method. We derive the a posteriori estimator using continuous reconstructions of the discrete solution. Combined with the relative entropy stability framework this yields computable error bounds for the entire space-stochastic discretization error. The estimator admits a splitting into a stochastic and a deterministic (space-time) part, allowing for a novel residual-based space-stochastic adaptive mesh refinement algorithm. We conclude with various numerical examples investigating the scaling properties of the residuals and illustrating the efficiency of the proposed adaptive algorithm

Frequency Characteristics of Visually Induced Motion Sickness

This article was published in the journal, Human Factors [Sage Publications / © Human Factors and Ergonomics Society.]. The definitive version is available at: http://dx.doi.org/10.1177/0018720812469046Objective: The aim of this study was to explore the frequency response of visually induced motion sickness (VIMS) for oscillating linear motion in the foreand- aft axis. Background: Simulators, virtual environments, and commercially available video games that create an illusion of self-motion are often reported to induce the symptoms seen in response to true motion. Often this human response can be the limiting factor in the acceptability and usability of such systems. Whereas motion sickness in physically moving environments is known to peak at an oscillation frequency around 0.2 Hz, it has recently been suggested that VIMS peaks at around 0.06 Hz following the proposal that the summed response of the visual and vestibular selfmotion systems is maximized at this frequency. Methods: We exposed 24 participants to random dot optical flow patterns simulating oscillating foreand- aft motion within the frequency range of 0.025 to 1.6 Hz. Before and after each 20-min exposure, VIMS was assessed with the Simulator Sickness Questionnaire. Also, a standard motion sickness scale was used to rate symptoms at 1-min intervals during each trial. Results: VIMS peaked between 0.2 and 0.4 Hz with a reducing effect at lower and higher frequencies. Conclusion: The numerical prediction of the “crossover frequency” hypothesis, and the design guidance curve previously proposed, cannot be accepted when the symptoms are purely visually induced. Application: In conditions in which stationary observers are exposed to optical flow that simulates oscillating fore-and-aft motion, frequencies around 0.2 to 0.4 Hz should be avoided

CLIC: a Two-Beam Multi-TeV e±e\pm Linear Collider

The CLIC study of a high-energy (0.5 - 5 TeV), high-luminosity (1034 - 1035 cm-2 sec-1) e+e- linear collider is presented. Beam acceleration using high frequency (30 GHz) normal-conducting structures operating at high accelerating fields (150 MV/m) significantly reduces the length and, in consequence, the cost of the linac. Using parameters derived from general scaling laws for linear colliders, the beam stability is shown to be similar to lower frequency designs in spite of the strong wake-field dependency on frequency. A new cost-effective and efficient drive beam generation scheme for RF power production by the so-called "Two-Beam Acceleration" method is described. It uses a thermionic gun and a fully-loaded normal-conducting linac operating at low frequency (937 MHz) to generate and accelerate the drive beam bunches, and RF multiplication by funnelling in compressor rings to produce the desired bunch structure. Recent 30 GHz hardware developments and CLIC Test Facility (CTF) results are described

A Comparative Study of Modern Inference Techniques for Structured Discrete Energy Minimization Problems

International audienceSzeliski et al. published an influential study in 2006 on energy minimization methods for Markov Random Fields (MRF). This study provided valuable insights in choosing the best optimization technique for certain classes of problems. While these insights remain generally useful today, the phenomenal success of random field models means that the kinds of inference problems that have to be solved changed significantly. Specifically , the models today often include higher order interactions, flexible connectivity structures, large label-spaces of different car-dinalities, or learned energy tables. To reflect these changes, we provide a modernized and enlarged study. We present an empirical comparison of more than 27 state-of-the-art optimization techniques on a corpus of 2,453 energy minimization instances from diverse applications in computer vision. To ensure reproducibility, we evaluate all methods in the OpenGM 2 framework and report extensive results regarding runtime and solution quality. Key insights from our study agree with the results of Szeliski et al. for the types of models they studied. However, on new and challenging types of models our findings disagree and suggest that polyhedral methods and integer programming solvers are competitive in terms of runtime and solution quality over a large range of model types