1,807 research outputs found
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
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
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
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
Observational constraints on annihilation sites in 1E 1740.7-2942 and Nova Muscae
The region of the Galactic center contains several sources which demonstrate
their activity at various wavelengths and particularly above several hundred
keV. Escape of positrons from such a source or several sources into the
interstellar medium, where they slow down and annihilate, can account for the
511 keV narrow line observed from this direction. 1E 1740.7-2942 object has
been proposed as the most likely candidate to be responsible for this variable
source of positrons. Besides, Nova Muscae shows a spectrum which is consistent
with Comptonization by a thermal plasma kT<100 keV in its hard X-ray part,
while a relatively narrow annihilation line observed by SIGMA on January 20-21,
1991 implies that positrons annihilate in a much colder medium.
We estimate the electron number density and the size of the emitting regions
suggesting that annihilation features observed by SIGMA from Nova Muscae and 1E
1740.7-2942 are due to the positron slowing down and annihilation in thermal
plasma. We show that in the case of Nova Muscae the observed radiation is
coming from a pair plasma stream, N(e+)~N(e-), rather than from a gas cloud. We
argue that two models are probably relevant to the 1E source: annihilation in
(hydrogen) plasma N(e+)<~N(e-) at rest, and annihilation in the pair plasma
stream, which involves matter from the source environment.Comment: 5 pages including 2 figures, latex, aipproc.sty, aipproc.cls,
epsfig.sty. To be published in Proc. of 4th Compton Symp., 1997 (27-30 April,
Williamsburg, Virginia
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