2,067 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

### 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

### Designing Algorithms for Optimization of Parameters of Functioning of Intelligent System for Radionuclide Myocardial Diagnostics

The influence of the number of complex components of Fast Fourier transformation in analyzing the polar maps of radionuclide examination of myocardium at rest and stress on the functional efficiency of the system of diagnostics of pathologies of myocardium was explored, and there were defined their optimum values in the information sense, which allows increasing the efficiency of the algorithms of forming the diagnostic decision rules by reducing the capacity of the dictionary of features of recognition.The information-extreme sequential cluster algorithms of the selection of the dictionary of features, which contains both quantitative and category features were developed and the results of their work were compared. The modificatios of the algorithms of the selection of the dictionary were suggested, which allows increasing both the search speed of the optimal in the information sense dictionary and reducing its capacity by 40 %. We managed to get the faultless by the training matrix decision rules, the accuracy of which is in the exam mode asymptotically approaches the limit.It was experimentally confirmed that the implementation of the proposed algorithm of the diagnosing system training has allowed to reduce the minimum representative volume of the training matrix from 300 to 81 vectors-implementations of the classes of recognition of the functional myocardium state

### Diagrammatic theory for Periodic Anderson Model: Stationary property of the thermodynamic potential

Diagrammatic theory for Periodic Anderson Model has been developed, supposing
the Coulomb repulsion of $f-$ localized electrons as a main parameter of the
theory. $f-$ electrons are strongly correlated and $c-$ conduction electrons
are uncorrelated. Correlation function for $f-$ and mass operator for $c-$
electrons are determined. The Dyson equation for $c-$ and Dyson-type equation
for $f-$ electrons are formulated for their propagators. The skeleton diagrams
are defined for correlation function and thermodynamic functional. The
stationary property of renormalized thermodynamic potential about the variation
of the mass operator is established. The result is appropriate as for normal
and as for superconducting state of the system.Comment: 12 pages, 10 figure

### Constraints on cosmic-ray propagation models from a global Bayesian analysis

Research in many areas of modern physics such as, e.g., indirect searches for
dark matter and particle acceleration in SNR shocks, rely heavily on studies of
cosmic rays (CRs) and associated diffuse emissions (radio, microwave, X-rays,
gamma rays). While very detailed numerical models of CR propagation exist, a
quantitative statistical analysis of such models has been so far hampered by
the large computational effort that those models require. Although statistical
analyses have been carried out before using semi-analytical models (where the
computation is much faster), the evaluation of the results obtained from such
models is difficult, as they necessarily suffer from many simplifying
assumptions, The main objective of this paper is to present a working method
for a full Bayesian parameter estimation for a numerical CR propagation model.
For this study, we use the GALPROP code, the most advanced of its kind, that
uses astrophysical information, nuclear and particle data as input to
self-consistently predict CRs, gamma rays, synchrotron and other observables.
We demonstrate that a full Bayesian analysis is possible using nested sampling
and Markov Chain Monte Carlo methods (implemented in the SuperBayeS code)
despite the heavy computational demands of a numerical propagation code. The
best-fit values of parameters found in this analysis are in agreement with
previous, significantly simpler, studies also based on GALPROP.Comment: 19 figures, 3 tables, emulateapj.sty. A typo is fixed. To be
published in the Astrophysical Journal v.728 (February 10, 2011 issue).
Supplementary material can be found at
http://www.g-vo.org/pub/GALPROP/GalpropBayesPaper

### Light-induced valley currents and magnetization in graphene rings

We study the non-equilibrium dynamics in a mesoscopic graphene ring excited
by picoseconds shaped electromagnetic pulses. We predict an ultrafast buildup
of charge polarization, currents and orbital magnetization. Applying the light
pulses identified here, non-equilibrium valley currents are generated in a
graphene ring threaded by a stationary magnetic flux. We predict a finite
graphene ring magnetization even for a vanishing charge current; the
magnetization emerges due to the light-induced difference of the valley
populations.Comment: 4 pages, 2 figures, submitted to Phys. Rev.

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