Microwave Response of V3Si Single Crystals: Evidence for Two-Gap Superconductivity

The investigation of the temperature dependences of microwave surface impedance and complex conductivity of V3Si single crystals with different stoichiometry allowed to observe a number of peculiarities which are in remarkable contradiction with single-gap Bardeen-Cooper-Schrieffer theory. At the same time, they can be well described by two-band model of superconductivity, thus strongly evidencing the existence of two distinct energy gaps with zero-temperature values Delta1~1.8Tc and Delta2~0.95Tc in V3Si.Comment: Submitted to Europhysics Letter

Three-band superconductivity and the order parameter that breaks time-reversal symmetry

We consider a model of multiband superconductivity, inspired by iron pnictides, in which three bands are connected via repulsive pair-scattering terms. Generically, three distinct superconducting states arise within such a model. Two of them are straightforward generalizations of the two-gap order parameter while the third one corresponds to a time-reversal symmetry breaking order parameter, altogether absent within the two-band model. Potential observation of such a genuinely frustrated state would be a particularly vivid manifestation of the repulsive interactions being at the root of iron-based high temperature superconductivity. We construct the phase diagram of this model and discuss its relevance to the iron pnictides family of high temperature superconductors. We also study the case of the Josephson coupling between a two-band s' (or extended s-wave) superconductor and a single-gap s-wave superconductor, and the associated phase diagram.Comment: 9 pages, 9 figures. Added discussion and references, one new figure (Fig. 3

GALPROP: modeling cosmic ray propagation and associated interstellar emissions

Research in many areas of modern physics and astrophysics such as, e.g., indirect searches for dark matter (DM), particle acceleration in SNR shocks, and the spectrum and origin of extragalactic gamma-ray background, rely heavily on studies of cosmic rays (CRs) and associated diffuse emissions. New or improved instrumentation to explore these open issues is ready or under development. A fleet of ground-based, balloon-borne, and spacecraft instruments measures many CR species, gamma rays, radio, and synchrotron emission. Exploiting the data collected by the scientific missions to the fullest requires reliable and detailed calculations using a numerical model. GALPROP is the current state-of-the-art numerical CR propagation code that has become a standard analysis tool in CR and diffuse gamma-ray research. It uses astrophysical information, nuclear and particle data as input to self-consistently predict CRs, gamma rays, synchrotron emission and other observables. This paper reviews recent GALPROP developments and results.Comment: Invited talk at 8th Workshop on Science with the New Generation of High Energy Gamma-ray Experiments: Gamma-ray Astrophysics in the Multimessenger Context (SciNeGHE2010, Trieste, Sept. 8-10, 2010); Il Nuovo Cimento C, v. 034, published onlin

An architecture for Olap-based enterprise-level Decision Support Systems

In this work it is considered that the strategic development of an enterprise is aimed at the improvement of the market position and financial status. Decision Support System for elaboration of development strategy of an enterprise is used. Suggested information support for algorithmic modules realization is based on OLAP technology.У статті пропонується розробляти стратегії розвитку підприємства на підставі поліпшення ринкової позиції підприємства та фінансового положення. Для розробки стратегії розвитку використовується система підтримки прийняття рішень. Пропонується використовувати OLAP-технології для інформаційної підтримки алгоритмічних модулів

Is the term "type-1.5 superconductivity" warranted by Ginzburg-Landau theory?

It is shown that within the Ginzburg-Landau (GL) approximation the order parameters Delta1(r, T) and Delta2(r, T) in two-band superconductors vary on the same length scale, the difference in the zero-T coherence lengths xi0_i ~vF_i/Delta_i(0), i = 1, 2 notwithstanding. This amounts to a single physical GL parameter kappa and the classic GL dichotomy: kappa < 1/sqrt(2) for type-I and kappa > 1/sqrt(2) for type-II.Comment: 5 pages, revised and extended version; previous title "Two-band superconductors near Tc" change

Interaction of strongly correlated electrons and acoustical phonons

We investigate the interaction of correlated electrons with acoustical phonons using the extended Hubbard-Holstein model in which both, the electron-phonon interaction and the on-site Coulomb repulsion are considered to be strong. The Lang-Firsov canonical transformation allows to obtain mobile polarons for which a new diagram technique and generalized Wick's theorem is used. This allows to handle the Coulomb repulsion between the electrons emerged into a sea of phonon fields (\textit{phonon clouds}). The physics of emission and absorption of the collective phonon-field mode by the polarons is discussed in detail. Moreover, we have investigated the different behavior of optical and acoustical phonon clouds when propagating through the lattice. In the strong-coupling limit of the electron-phonon interaction, and in the normal as well as in the superconducting phase, chronological thermodynamical averages of products of acoustical phonon-cloud operators can be expressed by one-cloud operator averages. While the normal one-cloud propagator has the form of a Lorentzian, the anomalous one is of Gaussian form and considerably smaller. Therefore, the anomalous electron Green's functions can be considered to be more important than corresponding polarons functions, i.e., pairing of electrons without phonon-clouds is easier to achieve than pairing of polarons with such clouds.Comment: : 28 pages, 9 figures, revtex4. Invited paper for a special issue of Low Temperature Physics dedicated to the 20th anniversary of HTS

A model for the high-energy emission of Cyg X-1

We construct a model of Cyg X-1 which describes self-consistently its emission from soft X-rays to MeV gamma rays. Instead of a compact pair-dominated gamma-ray emitting region, we consider a hot optically thin and spatially extended proton-dominated cloud surrounding the whole accretion disc. The gamma-ray emission is due to the bremsstrahlung, Comptonization, and positron annihilation, while the corona-disc model is retained for the X-ray emission. We show that the Cyg X-1 spectrum accumulated by OSSE, BATSE, and COMPTEL in 1991--95, as well as the HEAO-3 gamma1 and gamma2 spectra can be well fitted by our model. The derived parameters are in qualitative agreement with the picture in which the spectral changes are governed by the mass flow rate in the accretion disc. In this context, the hot outer corona could be treated as the advection-dominated flow co-existing with a standard thin accretion disc.Comment: 5 pages including 2 figures, latex, aipproc.sty, aipproc.cls, epsfig.sty. To be published in Proc. 4th Compton Symp., 1997 (27-30 April, Williamsburg, Virginia