Co atoms on Bi2_{2}Se3_{3} revealing a coverage dependent spin reorientation transition

We investigate Co nanostructures on Bi$_{2}$Se$_{3}$ by means of scanning tunneling microscopy and spectroscopy [STM/STS], X-ray absorption spectroscopy [XAS], X-ray magnetic dichroism [XMCD] and calculations using the density functional theory [DFT]. In the single adatom regime we find two different adsorption sites by STM. Our calculations reveal these to be the fcc and hcp hollow sites of the substrate. STS shows a pronounced peak for only one species of the Co adatoms indicating different electronic properties of both types. These are explained on the basis of our DFT calculations by different hybridizations with the substrate. Using XMCD we find a coverage dependent spin reorientation transition from easy-plane toward out-of-plane. We suggest clustering to be the predominant cause for this observation.Comment: 10 pages, 4 figure

Strong out-of-plane magnetic anisotropy of Fe adatoms on Bi2_2Te3_3

The electronic and magnetic properties of individual Fe atoms adsorbed on the surface of the topological insulator Bi$_2$Te$_3$(111) are investigated. Scanning tunneling microscopy and spectroscopy prove the existence of two distinct types of Fe species, while our first-principles calculations assign them to Fe adatoms in the hcp and fcc hollow sites. The combination of x-ray magnetic circular dichroism measurements and angular dependent magnetization curves reveals out-of-plane anisotropies for both species with anisotropy constants of $K_{\text{fcc}} = (10 \pm 4)$ meV/atom and $K_{\text{hcp}} = (8 \pm 4)$ meV/atom. These values are well in line with the results of calculations.Comment: 6 pages, 3 figure

Plasma Ejection from Magnetic Flares and the X-ray Spectrum of Cygnus X-1

The hard X-rays in Cyg X-1 and similar black hole sources are possibly produced in an active corona atop an accretion disk. We suggest that the observed weakness of X-ray reflection from the disk is due to bulk motion of the emitting hot plasma away from the reflector. A mildly relativistic motion causes aberration reducing X-ray emission towards the disk. This in turn reduces the reprocessed radiation from the disk and leads to a hard spectrum of the X-ray source. The resulting spectral index is Gamma=1.9B^{1/2} where B=gamma(1+beta) is the aberration factor for a bulk velocity beta=v/c. The observed Gamma=1.6 and the amount of reflection, R=0.3, in Cyg X-1 in the hard state can both be explained assuming a bulk velocity beta=0.3. We discuss one possible scenario: the compact magnetic flares are dominated by e+- pairs which are ejected away from the reflector by the pressure of the reflected radiation. We also discuss physical constraints on the disk-corona model and argue that the magnetic flares are related to magneto-rotational instabilities in the accretion disk.Comment: The final version, accepted for publication in ApJ Letter

Maximally localized Wannier functions in LaMnO3 within PBE+U, hybrid functionals, and partially self-consistent GW: an efficient route to construct ab-initio tight-binding parameters for e_g perovskites

Using the newly developed VASP2WANNIER90 interface we have constructed maximally localized Wannier functions (MLWFs) for the e_g states of the prototypical Jahn-Teller magnetic perovskite LaMnO3 at different levels of approximation for the exchange-correlation kernel. These include conventional density functional theory (DFT) with and without additional on-site Hubbard U term, hybrid-DFT, and partially self-consistent GW. By suitably mapping the MLWFs onto an effective e_g tight-binding (TB) Hamiltonian we have computed a complete set of TB parameters which should serve as guidance for more elaborate treatments of correlation effects in effective Hamiltonian-based approaches. The method-dependent changes of the calculated TB parameters and their interplay with the electron-electron (el-el) interaction term are discussed and interpreted. We discuss two alternative model parameterizations: one in which the effects of the el-el interaction are implicitly incorporated in the otherwise "noninteracting" TB parameters, and a second where we include an explicit mean-field el-el interaction term in the TB Hamiltonian. Both models yield a set of tabulated TB parameters which provide the band dispersion in excellent agreement with the underlying ab initio and MLWF bands.Comment: 30 pages, 7 figure

Finding and counting vertex-colored subtrees

The problems studied in this article originate from the Graph Motif problem introduced by Lacroix et al. in the context of biological networks. The problem is to decide if a vertex-colored graph has a connected subgraph whose colors equal a given multiset of colors $M$. It is a graph pattern-matching problem variant, where the structure of the occurrence of the pattern is not of interest but the only requirement is the connectedness. Using an algebraic framework recently introduced by Koutis et al., we obtain new FPT algorithms for Graph Motif and variants, with improved running times. We also obtain results on the counting versions of this problem, proving that the counting problem is FPT if M is a set, but becomes W[1]-hard if M is a multiset with two colors. Finally, we present an experimental evaluation of this approach on real datasets, showing that its performance compares favorably with existing software.Comment: Conference version in International Symposium on Mathematical Foundations of Computer Science (MFCS), Brno : Czech Republic (2010) Journal Version in Algorithmic

Relativistic parsec-scale jets: I. Particle acceleration

We develop a theory of particle acceleration inside relativistic rotating electron-positron force-free jets with spiral magnetic fields. We considered perturbation of the stationary magnetic field structure and found that acceleration takes place in the regions where the Alfven resonant condition with the eigenmodes in the jet is fulfilled, i.e. where the local Alfven speed is equal to the phase speed of an eigenmode. The acceleration mechanism is regular acceleration by the electric field of the electromagnetic wave, which is the eigenmode of the force-free cylindrical jet: particles drift out of the region of the large wave amplitude near the Alfven resonant surface and gain energy. Acceleration in the strong electric field near the Alfven resonance and synchrotron losses combine to form a power-law energy spectrum of ultra-relativistic electrons and positrons with index between 2 and 3 depending upon the initial energy of the injected particles. The power law distribution ranges from 10 MeV to 1000 MeV.Comment: 14 pages, 4 figures; Astron. Astrophys. in pres

MARS – A Fresh Look at Risk Assessment Modelling and Effective Safety Data Communication

Aviation industry’s safety management is strictly regulated since the introduction of Safety Management System (SMS) concept and its propagation through the industry and literature. Safety and Risk are in the focus of the constant research ranging from strictly technical and technological to organisational influence. Still effective and practical assessing and communicating the risk is a challenge thus diminishing optimal enhancing of organisations’ safety level. The paper will analyse the literature for the SMS in aviation for their ability to account for the complex dynamics from which safety in these kinds of systems tends to emerge – or not. Evolution of safety assessment methods and methodologies (including) the systemic models (RAG, FRAM, STAMP) will be presented. After this, it will introduce MARS as a model by describing the general features and their connection with biometrics for varied practical and theoretical reasons. Conceptual description and presentation of the model will be followed by presenting mathematical approach in the model. General presentation of mathematical approach will be applied to a case study involving aviation organisation and related data (related to Human, Technical and Environment factors). Finally one of these factors will be defined and analysed more in detail demonstrating the effectiveness and application of the MARS on the operational data

Conversion of relativistic pair energy into radiation in the jets of active galactic nuclei

It is generally accepted that relativistic jet outflows power the nonthermal emission from active galactic nuclei (AGN). The composition of these jets -- leptonic versus hadronic -- is still under debate. We investigate the microphysical details of the conversion process of the kinetic energy in collimated relativistic pair outflows into radiation through interactions with the ambient interstellar medium. Viewed from the coordinate system comoving with the pair outflow, the interstellar protons and electrons represent a proton-electron beam propagating with relativistic speed in the pair plasma. We demonstrate that the beam excites both electrostatic and low-frequency magnetohydrodynamic Alfven-type waves via a two-stream instability in the pair background plasma, and we calculate the time evolution of the distribution functions of the beam particles and the generated plasma wave turbulence power spectra. For standard AGN jet outflow and environment parameters we show that the initial beam distributions of interstellar protons and electrons quickly relax to plateau-distributions in parallel momentum, transferring thereby one-half of the initial energy density of the beam particles to electric field fluctuations of the generated electrostatic turbulence. On considerably longer time scales, the plateaued interstellar electrons and protons will isotropise by their self-generated transverse turbulence and thus be picked-up in the outflow pair plasma. These longer time scales are also characteristic for the development of transverse hydromagnetic turbulence from the plateaued electrons and protons. This hydromagnetic turbulence upstream and downstream is crucial for diffusive shock acceleration to operate at external or internal shocks associated with pair outflows.Comment: A&A in pres

Safety Assessment For An ATM System Change: A Methodology For The ANSPs

Air Traffic Management (ATM) is a continuously evolving process, where many current system elements derive from a time when ATM characteristics were very different from today. Nowadays, the provision of ATM services has to design new solutions and adapt to new scenarios. Although ESARR 4 and EU Regulation 1035/2011 define the need of evaluating ATM system risks before implementing any change, they do not define a practical tool to support the decision-makers. The aim of this paper is to fill this gap, proposing a systematic methodology; the Preliminary System Safety Assessment Tool (PSSA-T) capable of helping the decision makers in evaluating safety implications due to system changes. PSSA-T relies on the definition of two Indexes, which have been built according to the Aerospace Performance Factor (APF) methodology, and allow safety assessment of any proposed change. In detail, the former Index compares the evolutionary scenario with the current one and the latter evaluates the evolutionary scenario in which there is a failure of intervention, in the hypothesis the system change has been implemented already. A preliminary study about the change from Flight Progress Strip (FPS) to the Electronic-FPS clarifies the outcome of the study