Measurement of the electronic compressibility of bilayer graphene

We present measurements of the electronic compressibility, $K$, of bilayer graphene in both zero and finite magnetic fields up to 14 T, and as a function of both the carrier density and electric field perpendicular to the graphene sheet. The low energy hyperbolic band structure of bilayer graphene is clearly revealed in the data, as well as a sizable asymmetry between the conduction and valence bands. A sharp increase in $K^{-1}$ near zero density is observed for increasing electric field strength, signaling the controlled opening of a gap between these bands. At high magnetic fields, broad Landau level (LL) oscillations are observed, directly revealing the doubled degeneracy of the lowest LL and allowing for a determination of the disorder broadening of the levels.Comment: 5 pages, 3 figures; final version for publicatio

RXTE and ASCA Constraints on Non-thermal Emission from the A2256 Galaxy Cluster

An 8.3 hour observation of the Abell 2256 galaxy cluster using the Rossi X-ray Timing Explorer proportional counter array produced a high quality spectrum in the 2 - 30 keV range. Joint fitting with the 0.7 - 11 keV spectrum obtained with the Advanced Satellite for Astrophysics and Cosmology gas imaging spectrometer gives an upperlimit of 2.3x10^-7 photons/cm^2/sec/keV for non-thermal emission at 30 keV. This yields a lower limit to the mean magnetic field of 0.36 micro Gauss (uG) and an upperlimit of 1.8x10^-13 ergs/cm^3 for the cosmic-ray electron energy density. The resulting lower limit to the central magnetic field is ~1 - 3 uG While a magnetic field of ~0.1 - 0.2 uG can be created by galaxy wakes, a magnetic field of several uG is usually associated with a cooling flow or, as in the case of the Coma cluster, a subcluster merger. However, for A2256, the evidence for a merger is weak and the main cluster shows no evidence of a cooling flow. Thus, there is presently no satisfactory hypothesis for the origin of an average cluster magnetic field as high as >0.36 uG in the A2256 cluster.Comment: 8 pages, Astrophysical Journal (in press

Quantum Hall Effect and Semimetallic Behavior of Dual-Gated ABA-Stacked Trilayer Graphene

The electronic structure of multilayer graphenes depends strongly on the number of layers as well as the stacking order. Here we explore the electronic transport of purely ABA-stacked trilayer graphenes in a dual-gated field-effect device configuration. We find that both the zero-magnetic-field transport and the quantum Hall effect at high magnetic fields are distinctly different from the monolayer and bilayer graphenes, and that they show electron-hole asymmetries that are strongly suggestive of a semimetallic band overlap. When the ABA trilayers are subjected to an electric field perpendicular to the sheet, Landau level splittings due to a lifting of the valley degeneracy are clearly observed.Comment: 5 figure

Radio jet refraction in galactic atmospheres with static pressure gradients

A theory of double radio sources which have a 'Z' or 'S' morphology is proposed, based on the refraction of radio jets in the extended atmosphere of an elliptical galaxy. The model describes a collimated jet of supersonic material bending self-consistently under the influence of external static pressure gradients. Gravity and magnetic fields are neglected in the simplest case except insofar as they determine the static pressure distribution. The calculation is a straightforward extension of a method used to calculate a ram-pressure model for twin radio trails ('C' morphology). It may also be described as a continuous-jet version of a buoyancy model proposed in 1973. The model has the added virtue of invoking a galactic atmosphere similar to those already indicated by X-ray measurements of some other radio galaxies and by models for the collimation of other radio jets

Repairing misclassifications in neural networks using limited data

We present a novel and computationally efficient method for repairing a feed-forward neural network with respect to a finite set of inputs that are misclassified. The method assumes no access to the training set. We present a formal characterisation for repairing the neural network and study its resulting properties in terms of soundness and minimality. We introduce a gradient-based algorithm that performs localised modifications to the network's weights such that misclassifications are repaired while marginally affecting network accuracy on correctly classified inputs. We introduce an implementation, I-REPAIR, and show it is able to repair neural networks while reducing accuracy drops by up to 90% when compared to other state-of-the-art approaches for repair

An X-ray Survey of Galaxies in Pairs

Results are reported from the first survey of X-ray emission from galaxies in pairs. The sample consists of fifty-two pairs of galaxies from the Catalog of Paired Galaxies Karachentsev (1972) whose coordinates overlap ROSAT Position Sensitive Proportional Counter pointed observations. The mean observed log l_x for early-type pairs is 41.35 +/-0.21 while the mean log l_x predicted using the l_x-l_b relationship for isolated early-type galaxies is 42.10 +/-0.19. With 95% confidence, the galaxies in pairs are underluminous in the X-ray, compared to isolated galaxies, for the same l_b. A significant fraction of the mixed pair sample also appear similarly underluminous. A spatial analysis shows that the X-ray emission from pairs of both types typically has an extent of ~10 - 50 kpc, much smaller than group intergalactic medium and thus likely originates from the galaxies. CPG 564, the most X-ray luminous early-type pair, 4.7x10^42 ergs/sec, is an exception. The extent of it's X-ray emission, >169 kpc, and HWHM, ~80 kpc, is comparable to that expected from an intergalactic medium. The sample shows only a weak correlation, ~81% confidence, between l_x and l_b, presumably due to variations in gas content within the galaxies. No correlation between l_x and the pair velocity difference, separation, or far-infrared luminosity is found though the detection rate is low, 22%.Comment: 40 pages, 6 jpg figures, ApJ (in press