The EVN view of the highly variable TeV active galaxy IC 310

Very-high-energy $\gamma$-ray observations of the active galaxy IC 310 with the MAGIC telescopes have revealed fast variability with doubling time scales of less than 4.8min. This implies that the emission region in IC 310 is smaller than 20% of the gravitational radius of the central supermassive black hole with a mass of $3\times 10^8 M_\odot$, which poses serious questions on the emission mechanism and classification of this enigmatic object. We report on the first quasi-simultaneous multi-frequency VLBI observations of IC 310 conducted with the EVN. We find a blazar-like one-sided core-jet structure on parsec scales, constraining the inclination angle to be less than $\sim 20^\circ$ but very small angles are excluded to limit the de-projected length of the large-scale radio jet.Comment: 4 pages, proceedings of the 12th European VLBI Network Symposium and Users Meeting - EVN 2014, 7-10 October 2014, Cagliari, Italy. Published online in PoS, ID.10

Prediction of subgap states in Zn- and Sn-based oxides using various exchange-correlation functionals

We present a density-functional-theory analysis of crystalline and amorphous Zn- and Sn-based oxide systems which focuses on the electronic defect states within the band gap. A comparison of these electronic levels reveals that the hybrid functionals PBE0, HSE06, or B3LYP agree with a self-interaction corrected (SIC) local-density-approximation functional on occupied defect levels when similar treatments of the self-interaction are considered. However, for unoccupied levels, the hybrid functionals and the SIC approach lead to very different predictions. We show that a prerequisite for the determination of the energetic position of subgap states in these oxides is that a functional needs to predict correctly the electronic band structure over a wide energy range and not just close to the band gap. We conclude that for accurate defect levels, an adequate treatment of the self-interaction problem is required especially in the presence of nearby metal-metal interactions.Financial support for this work was provided by the European Commission through Contract No. NMP3-LA-2010-246334 (ORAMA). The calculations at Cambridge were performed using the High Performance Computing Facility, Darwin, and also the UK national high performance computing service ARCHER, for which access was obtained via the UKCP consortium and funded by EPSRC Grant No. EP/K014560/1.Phys. Rev. B 90, 195142 – Published 21 November 2014 ©2014 American Physical Society, http://dx.doi.org/10.1103/PhysRevB.90.19514

First-principles analysis of the interplay between electronic structure and volume change in colquiriite compounds during Li intercalation

A main source of capacity fading in lithium-ion batteries is the degradation of the active cathode materials caused by the series of volume changes during charge and discharge cycles. The quaternary colquiriite-type fluorides Li$_x$CaFeF$\mathrm{_6}$ and Li$_x$CaCoF$\mathrm{_6}$ were reported to have negligible volume changes in specific Li concentration ranges, making the underlying colquiriite structure a promising candidate for so-called zero-strain behavior. Using first-principles electronic structure calculations based on density functional theory with a Hubbard-$U$ correlation correction on the transition-metal ions, we systematically investigate the equilibrium volumes of the colquiriite-type fluorides Li$_x$CaMF$\mathrm{_6}$ with M =Ti, V, Cr, Mn, Fe, Co, and Ni at the Li concentrations $x$=0, 1, and 2. We elucidate the connection between the total volume of the structures and the local volumes of fluorine coordinated octahedra around the cations, and we find trends along the series of the 3d transition-metal elements. In the lithiation step from $x$=1 to $x$=2 we find volume changes of about 10 %, and we discuss the discrepancy to the experimentally reported smaller value for Li$_x$CaFeF$\mathrm{_6}$. From $x$=0 to $x$=1 we describe the compensating structural mechanisms that lead to an exceptionally small volume change of Li$_x$CaMnF$\mathrm{_6}$. This compound is therefore a particularly promising zero-strain cathode material.Comment: 13 pages, 9 Figure

Electronic correlations in vanadium chalcogenides: BaVSe3 versus BaVS3

Albeit structurally and electronically very similar, at low temperature the quasi-one-dimensional vanadium sulfide BaVS3 shows a metal-to-insulator transition via the appearance of a charge-density-wave state, while BaVSe3 apparently remains metallic down to zero temperature. This different behavior upon cooling is studied by means of density functional theory and its combination with the dynamical mean-field theory and the rotationally-invariant slave-boson method. We reveal several subtle differences between these chalcogenides that provide indications for the deviant behavior of BaVSe3 at low temperature. In this regard, a smaller Hubbard U in line with an increased relevance of the Hund's exchange J plays a vital role.Comment: 16 pages, 11 figures, published versio

The prismatic Sigma 3 (10-10) twin bounday in alpha-Al2O3 investigated by density functional theory and transmission electron microscopy

The microscopic structure of a prismatic $\Sigma 3$ $(10\bar{1}0)$ twin boundary in \aal2o3 is characterized theoretically by ab-initio local-density-functional theory, and experimentally by spatial-resolution electron energy-loss spectroscopy in a scanning transmission electron microscope (STEM), measuring energy-loss near-edge structures (ELNES) of the oxygen $K$-ionization edge. Theoretically, two distinct microscopic variants for this twin interface with low interface energies are derived and analysed. Experimentally, it is demonstrated that the spatial and energetical resolutions of present high-performance STEM instruments are insufficient to discriminate the subtle differences of the two proposed interface variants. It is predicted that for the currently developed next generation of analytical electron microscopes the prismatic twin interface will provide a promising benchmark case to demonstrate the achievement of ELNES with spatial resolution of individual atom columns

A new Determination of the Extragalactic Background of Diffuse Gamma Rays taking into account Dark Matter Annihilation

The extragalactic background (EGB) of diffuse gamma rays can be determined by subtracting the Galactic contribution from the data. This requires a Galactic model (GM) and we include for the first time the contribution of dark matter annihilation (DMA), which was previously proposed as an explanation for the EGRET excess of diffuse Galactic gamma rays above 1 GeV. In this paper it is shown that the newly determined EGB shows a characteristic high energy bump on top of a steeply falling soft contribution. The bump is shown to be compatible with a contribution from an extragalactic DMA signal from weakly interacting massive particles (WIMPs) with a mass between 50 and 100 GeV in agreement with the EGRET excess of the Galactic diffuse gamma rays and in disagreement with earlier analysis. The remaining soft contribution of the EGB is shown to resemble the spectra of the observed point sources in our Galaxy.Comment: 7 pages, 4 figures. Accepted by A&A, made Fig. 4 and table 1 consisten

The Sigma 13 (10-14) twin in alpha-Al2O3: A model for a general grain boundary

The atomistic structure and energetics of the Sigma 13 (10-14)[1-210] symmetrical tilt grain boundary in alpha-Al2O3 are studied by first-principles calculations based on the local-density-functional theory with a mixed-basis pseudopotential method. Three configurations, stable with respect to intergranular cleavage, are identified: one Al-terminated glide-mirror twin boundary, and two O-terminated twin boundaries, with glide-mirror and two-fold screw-rotation symmetries, respectively. Their relative energetics as a function of axial grain separation are described, and the local electronic structure and bonding are analysed. The Al-terminated variant is predicted to be the most stable one, confirming previous empirical calculations, but in contrast with high-resolution transmission electron microscopy observations on high-purity diffusion-bonded bicrystals, which resulted in an O-terminated structure. An explanation of this discrepancy is proposed, based on the different relative energetics of the internal interfaces with respect to the free surfaces

Parallel Search with no Coordination

We consider a parallel version of a classical Bayesian search problem. $k$ agents are looking for a treasure that is placed in one of the boxes indexed by $\mathbb{N}^+$ according to a known distribution $p$. The aim is to minimize the expected time until the first agent finds it. Searchers run in parallel where at each time step each searcher can "peek" into a box. A basic family of algorithms which are inherently robust is \emph{non-coordinating} algorithms. Such algorithms act independently at each searcher, differing only by their probabilistic choices. We are interested in the price incurred by employing such algorithms when compared with the case of full coordination. We first show that there exists a non-coordination algorithm, that knowing only the relative likelihood of boxes according to $p$, has expected running time of at most $10+4(1+\frac{1}{k})^2 T$, where $T$ is the expected running time of the best fully coordinated algorithm. This result is obtained by applying a refined version of the main algorithm suggested by Fraigniaud, Korman and Rodeh in STOC'16, which was designed for the context of linear parallel search.We then describe an optimal non-coordinating algorithm for the case where the distribution $p$ is known. The running time of this algorithm is difficult to analyse in general, but we calculate it for several examples. In the case where $p$ is uniform over a finite set of boxes, then the algorithm just checks boxes uniformly at random among all non-checked boxes and is essentially $2$ times worse than the coordinating algorithm.We also show simple algorithms for Pareto distributions over $M$ boxes. That is, in the case where $p(x) \sim 1/x^b$ for $0< b < 1$, we suggest the following algorithm: at step $t$ choose uniformly from the boxes unchecked in ${1, . . . ,min(M, \lfloor t/\sigma\rfloor)}$, where $\sigma = b/(b + k - 1)$. It turns out this algorithm is asymptotically optimal, and runs about $2+b$ times worse than the case of full coordination