1,502 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
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 localized electrons as a main parameter of the
theory. electrons are strongly correlated and conduction electrons
are uncorrelated. Correlation function for and mass operator for
electrons are determined. The Dyson equation for and Dyson-type equation
for 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
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
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
Modelling cosmic rays and gamma rays in the Galaxy
An extensive program for the calculation of galactic cosmic-ray propagation
has been developed. This is a continuation of the work described in Strong &
Youssefi (1995). The main motivation for developing this code is the prediction
of diffuse Galactic gamma rays for comparison with data from the CGRO
instruments EGRET, COMPTEL, and OSSE. The basic spatial propagation mechanisms
are (momentum-dependent) diffusion, convection, while in momentum space energy
loss and diffusive reacceleration are treated. Primary and secondary nucleons,
primary and secondary electrons, and secondary positrons are included.
Fragmentation and energy losses are computed using realistic distributions for
the interstellar gas and radiation fields.
This study indicates that it is possible to construct a model satisfying a
wide range of observational constraints and provides a basis for future
developments.Comment: 5 pages including 7 figures, latex, aipproc.sty, aipproc.cls,
epsfig.sty. To be published in Proc. 4th Compton Symp., 1997 (27-30 April,
Williamsburg, Virginia). Details can be found at
http://www.gamma.mpe-garching.mpg.de/~aws/aws.htm
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