162 research outputs found
Observation of the Holstein shift in high superconductors with thermal modulation reflectometry
We use the experimental technique of thermal modulation reflectometry to
study the relatively small temperature dependence of the optical conductivity
of superconductors. Due to a large cancellation of systematic errors, this
technique is shown to a be very sensitive probe of small changes in
reflectivity. We analyze thermal modulation reflection spectra of single
crystals and epitaxially grown thin films of YBaCuO and
obtain the function in the normal state, as well as
the superconductivity induced changes in reflectivity. We present detailed
model calculations, based on the Eliashberg-Migdal extension of the BCS model,
which show good qualitative and quantitative agreement with the experimental
spectra. VSGD.93.12.thComment: 6 pages, figures on request. Revtex, version 2, Materials Science
Center Internal Report Number VSGD.93.12.t
Can one extract the electron-phonon-interaction from tunneling data in case of the multigap superconductor MgB?
In the present work we calculate the tunneling density of states (DOS) of
MgB% for different tunneling directions by directly solving the two-band
Eliashberg equations (EE) in the real-axis formulation. This procedure reveals
the fine structures of the DOS due to the optical phonons. Then we show that
the numeric inversion of the standard \emph{single-band} EE (the only available
method), when applied to the \emph{two-band} DOS of MgB, may lead to
wrong estimates of the strength of certain phonon branches (e.g. the )
in the extracted electron-phonon spectral function . The
fine structures produced by the two-band interaction at energies between 20 and
100 meV turn out to be clearly observable only for tunneling along the
planes, when the extracted contains the combination
\textbf{+}, together with a minor \textbf{+} component. Only in this case
it is possible to extract information on the -band contribution to the
spectral functions. For any other tunneling direction, the -band
contribution (which does not determine the superconducting properties of
MgB) is dominant and almost coincides with the whole
for tunneling along the c axis. Our results are compared with recent
experimental tunneling and point-contact data.Comment: 5 pages, 3 figures. Submitted to Phys. Rev. B (Brief Reports
Surface effects in multiband superconductors. Application to MgB
Metals with many bands at the Fermi level can have different band dependent
gaps in the superconducting state. The absence of translational symmetry at an
interface can induce interband scattering and modify the superconducting
properties. We dicuss the relevance of these effects to recent experiments in
MgB
Enhancement of the upper critical field by nonmagnetic impurities in dirty two-gap superconductors
Quasiclassic Uzadel equations for two-band superconductors in the dirty limit
with the account of both intraband and interband scattering by nonmagnetic
impurities are derived for any anisotropic Fermi surface. From these equations
the Ginzburg-Landau equations, and the critical temperature are obtained.
An equation for the upper critical field, which determines both the temperature
dependence of and the orientational dependence of
as a function of the angle between and the c-axis is
obtained. It is shown that the shape of the curve essentially
depends on the ratio of the intraband electron diffusivities and ,
and can be very different from the standard one-gap dirty limit theory. In
particular, the value can considerably exceed ,
which can have important consequences for applications of . A scaling
relation is proposed which enables one to obtain the angular dependence of
from the equation for at . It is shown
that, depending on the relation between and , the ratio of the upper
critical field for and can both increase and decrease as the temperature decreases. Implications
of the obtained results for are discussed
Astrometry at the RTT150 telescope within the international collaboration between KSU (Russia), TUG (Turkey), and NAO (Ukraine)
Modern astrometric projects require precise positional measurements of objects down to magnitude 20mâ22m. For ground-based observations, this is possible by the use of astrographs with apertures 1 meter or more with precise tracking using long exposures and precise timing. The multifunctional astronomical complex RTT150 (the 1.5-m RussianâTurkish Telescope) is appropriate for such purposes
Surface plasmon-polariton resonance at diffraction of THz radiation on semiconductor gratings
Resonance diffraction of THz hidrogen cyanide laser radiation on a semiconductor (InSb) grating is studied both
experimentally and theoretically. The specular reflectivity suppression due to the resonance excitation of the THz
surface plasmon-polariton is observed on a pure semiconductor grating and on semiconductor gratings covered with
a thin dielectric layer. The dielectric coating of the grating results in the resonance shift and widening depending
both on the layer thickness and dielectric properties. A simple analytical theory of the resonance diffraction on rather
shallow gratings covered with a dielectric layer is presented, and the results are in a good accordance with the
experimental data. Analytical expressions for the resonance shift and broadening are essential for the resonance
properties understanding and useful for sensing data interpretation of the agents deposited on the grating surface
Phonon-mediated anisotropic superconductivity in the Y and Lu nickel borocarbides
We present scanning tunneling spectroscopy and microscopy measurements at low
temperatures in the borocarbide materials RNi2B2C (R=Y, Lu). The characteristic
strong coupling structure due to the pairing interaction is unambiguously
resolved in the superconducting density of states. It is located at the
superconducting gap plus the energy corresponding to a phonon mode identified
in previous neutron scattering experiments. These measurements also show that
this mode is coupled to the electrons through a highly anisotropic
electron-phonon interaction originated by a nesting feature of the Fermi
surface. Our experiments, from which we can extract a large electron-phonon
coupling parameter lambda (between 0.5 and 0.8), demonstrate that this
anisotropic electron-phonon coupling has an essential contribution to the
pairing interaction. The tunneling spectra show an anisotropic s-wave
superconducting gap function.Comment: 5 pages, 3 figure
Electron-Phonon Properties of Pnictide Superconductors
In this paper we discuss the normal and superconducting state properties of
two pnictide superconductors, LaOFeAs and LaONiAs, using Migdal-Eliashberg
theory and density functional perturbation theory. For pure LaOFeAs, the
calculated electron-phonon coupling constant and
logarithmic-averaged frequency , give a maximum of 0.8
K, using the standard Migdal-Eliashberg theory. Inclusion of multiband effects
increases the Tc only marginally. To reproduce the experimental , a 5-6
times larger coupling constant would be needed. Our results indicate that
standard electron-phonon coupling is not sufficient to explain
superconductivity in the whole family of Fe-As based superconductors. At the
same time, the electron-phonon coupling in Ni-As based compounds is much
stronger and its normal and superconducting state properties can be well
described by standard Migdal-Eliashberg theory.Comment: Contribution to the special issue of the Physica C on Pnicitide
superconductor
Search for direct production of charginos and neutralinos in events with three leptons and missing transverse momentum in âs = 7 TeV pp collisions with the ATLAS detector
A search for the direct production of charginos and neutralinos in final states with three electrons or muons and missing transverse momentum is presented. The analysis is based on 4.7 fbâ1 of protonâproton collision data delivered by the Large Hadron Collider and recorded with the ATLAS detector. Observations are consistent with Standard Model expectations in three signal regions that are either depleted or enriched in Z-boson decays. Upper limits at 95% confidence level are set in R-parity conserving phenomenological minimal supersymmetric models and in simplified models, significantly extending previous results
Jet size dependence of single jet suppression in lead-lead collisions at sqrt(s(NN)) = 2.76 TeV with the ATLAS detector at the LHC
Measurements of inclusive jet suppression in heavy ion collisions at the LHC
provide direct sensitivity to the physics of jet quenching. In a sample of
lead-lead collisions at sqrt(s) = 2.76 TeV corresponding to an integrated
luminosity of approximately 7 inverse microbarns, ATLAS has measured jets with
a calorimeter over the pseudorapidity interval |eta| < 2.1 and over the
transverse momentum range 38 < pT < 210 GeV. Jets were reconstructed using the
anti-kt algorithm with values for the distance parameter that determines the
nominal jet radius of R = 0.2, 0.3, 0.4 and 0.5. The centrality dependence of
the jet yield is characterized by the jet "central-to-peripheral ratio," Rcp.
Jet production is found to be suppressed by approximately a factor of two in
the 10% most central collisions relative to peripheral collisions. Rcp varies
smoothly with centrality as characterized by the number of participating
nucleons. The observed suppression is only weakly dependent on jet radius and
transverse momentum. These results provide the first direct measurement of
inclusive jet suppression in heavy ion collisions and complement previous
measurements of dijet transverse energy imbalance at the LHC.Comment: 15 pages plus author list (30 pages total), 8 figures, 2 tables,
submitted to Physics Letters B. All figures including auxiliary figures are
available at
http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HION-2011-02
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