Electrostatic confinement of electrons in an integrable graphene quantum dot

We compare the conductance of an undoped graphene sheet with a small region subject to an electrostatic gate potential for the cases that the dynamics in the gated region is regular (disc-shaped region) and classically chaotic (stadium). For the disc, we find sharp resonances that narrow upon reducing the area fraction of the gated region. We relate this observation to the existence of confined electronic states. For the stadium, the conductance looses its dependence on the gate voltage upon reducing the area fraction of the gated region, which signals the lack of confinement of Dirac quasiparticles in a gated region with chaotic classical electron dynamics.Comment: 4 pages, 4 figures; [v2] Added discussion of large aspect ratio

Signatures of Klein tunneling in disordered graphene p-n-p junctions

We present a method for obtaining quantum transport properties in graphene that uniquely combines three crucial features: microscopic treatment of charge disorder, fully quantum mechanical analysis of transport, and the ability to model experimentally relevant system sizes. As a pertinent application we study the disorder dependence of Klein tunneling dominated transport in p-n-p junctions. Both the resistance and the Fano factor show broad resonance peaks due to the presence of quasi bound states. This feature is washed out by the disorder when the mean free path becomes of the order of the distance between the two p-n interfaces.Comment: 4 pages, 4 figure

Demonstration of one-parameter scaling at the Dirac point in graphene

We numerically calculate the conductivity $\sigma$ of an undoped graphene sheet (size $L$) in the limit of vanishingly small lattice constant. We demonstrate one-parameter scaling for random impurity scattering and determine the scaling function $\beta(\sigma)=d\ln\sigma/d\ln L$. Contrary to a recent prediction, the scaling flow has no fixed point ($\beta>0$) for conductivities up to and beyond the symplectic metal-insulator transition. Instead, the data supports an alternative scaling flow for which the conductivity at the Dirac point increases logarithmically with sample size in the absence of intervalley scattering -- without reaching a scale-invariant limit.Comment: 4 pages, 5 figures; v2: introduction expanded, data for Gaussian model extended to larger system sizes to further demonstrate single parameter scalin

On the Veldkamp Space of GQ(4, 2)

The Veldkamp space, in the sense of Buekenhout and Cohen, of the generalized quadrangle GQ(4, 2) is shown not to be a (partial) linear space by simply giving several examples of Veldkamp lines (V-lines) having two or even three Veldkamp points (V-points) in common. Alongside the ordinary V-lines of size five, one also finds V-lines of cardinality three and two. There, however, exists a subspace of the Veldkamp space isomorphic to PG(3, 4) having 45 perps and 40 plane ovoids as its 85 V-points, with its 357 V-lines being of four distinct types. A V-line of the first type consists of five perps on a common line (altogether 27 of them), the second type features three perps and two ovoids sharing a tricentric triad (240 members), whilst the third and fourth type each comprises a perp and four ovoids in the rosette centered at the (common) center of the perp (90). It is also pointed out that 160 non-plane ovoids (tripods) fall into two distinct orbits -- of sizes 40 and 120 -- with respect to the stabilizer group of a copy of GQ(2, 2); a tripod of the first/second orbit sharing with the GQ(2, 2) a tricentric/unicentric triad, respectively. Finally, three remarkable subconfigurations of V-lines represented by fans of ovoids through a fixed ovoid are examined in some detail.Comment: 6 pages, 7 figures; v2 - slightly polished, subsection on fans of ovoids and three figures adde

Exponential sensitivity to dephasing of electrical conduction through a quantum dot

According to random-matrix theory, interference effects in the conductance of a ballistic chaotic quantum dot should vanish $\propto(\tau_{\phi}/\tau_{D})^{p}$ when the dephasing time $\tau_{\phi}$ becomes small compared to the mean dwell time $\tau_{D}$. Aleiner and Larkin have predicted that the power law crosses over to an exponential suppression $\propto\exp(-\tau_{E}/\tau_{\phi})$ when $\tau_{\phi}$ drops below the Ehrenfest time $\tau_{E}$. We report the first observation of this crossover in a computer simulation of universal conductance fluctuations. Their theory also predicts an exponential suppression $\propto\exp(-\tau_{E}/\tau_{D})$ in the absence of dephasing -- which is not observed. We show that the effective random-matrix theory proposed previously for quantum dots without dephasing explains both observations.Comment: 4 pages, 4 figure

The Painting Industries of Antwerp and Amsterdam, 1500−1700: A Data Perspective

This study presents a data driven comparative analysis of the painting industries in sixteenth and seventeenth century Antwerp and Amsterdam. The popular view of the development of these two artistic centers still holds that Antwerp flourished in the sixteenth century and was succeeded by Amsterdam after the former’s recapturing by the Spanish in 1585. However, a demographic analysis of the number of painters active in Antwerp and Amsterdam shows that Antwerp recovered relatively quickly after 1585 and that it remained the leading artistic center in the Low Countries, only to be surpassed by Amsterdam in the 1650’s. An analysis of migration patterns and social networks shows that painters in Antwerp formed a more cohesive group than painters in Amsterdam. As a result, the two cities responded quite differently to internal and external market shocks. Data for this study are taken from ECARTICO, a database and a linked data web resource containing structured biographical data on over 9100 painters working in the Low Countries until circa 1725

Welfarism vs. extra-welfarism

'Extra-welfarism' has received some attention in health economics, yet there is little consensus on what distinguishes it from more conventional 'welfarist economics'. In this paper, we seek to identify the characteristics of each in order to make a systematic comparison of the ways in which they evaluate alternative social states. The focus, though this is not intended to be exclusive, is on health. Specifically, we highlight four areas in which the two schools differ: (i) the outcomes considered relevant in an evaluation; (ii) the sources of valuation of the relevant outcomes; (iii) the basis of weighting of relevant outcomes and (iv) interpersonal comparisons. We conclude that these differences are substantive. (C) 2007 Elsevier B.V. All rights reserved