3,001 research outputs found
Specific heat of single crystal MgB_2: a two-band superconductor with two different anisotropies
Heat-capacity measurements of a 39 microgramm MgB_2 single crystal in fields
up to 14 T and below 3 K allow the determination of the low-temperature linear
term of the specific heat, its field dependence and its anisotropy. Our results
are compatible with two-band superconductivity, the band carrying the small gap
being isotropic, that carrying the large gap having an anisotropy of ~ 5. Three
different upper critical fields are thus needed to describe the superconducting
state of MgB2.Comment: 4 pages, 4 figures - V2: Bibliography updated and some typo
corrected. One reference added - V3: version accepted for publication in PRL,
changes made in the tex
Fermi surface topology and vortex state in MgB2
Based on a detailed modeling of the Fermi surface topology of MgB2 we
calculated the anisotropy of the upper critical field Bc2 within the two gap
model. The sigma-band is modeled as a distorted cylinder and the pi-band as a
half-torus, with parameters determined from bandstructure calculations. Our
results show that the unusual strong temperature dependence of the Bc2
anisotropy, that has been observed recently, can be understood due to the small
c-axis dispersion of the cylindrical Fermi surface sheets and the small
interband pairing interaction as obtained from bandstructure calculations. We
calculate the magnetic field dependence of the density of states within the
vortex state for field in c-axis direction and compare with recent measurements
of the specific heat on MgB2 single crystals.Comment: 2 pages, 2 figure
Non-stationary Rayleigh-Taylor instability in supernovae ejecta
The Rayleigh-Taylor instability plays an important role in the dynamics of
several astronomical objects, in particular, in supernovae (SN) evolution. In
this paper we develop an analytical approach to study the stability analysis of
spherical expansion of the SN ejecta by using a special transformation in the
co-moving coordinate frame. We first study a non-stationary spherical expansion
of a gas shell under the pressure of a central source. Then we analyze its
stability with respect to a no radial, non spherically symmetric perturbation
of the of the shell. We consider the case where the polytropic constant of the
SN shell is and we examine the evolution of a arbitrary shell
perturbation. The dispersion relation is derived. The growth rate of the
perturbation is found and its temporal and spatial evolution is discussed. The
stability domain depends on the ejecta shell thickness, its acceleration, and
the perturbation wavelength.Comment: 16 page
High-Tech Kit—The set of advanced activities from the MOSEM project
One of the most tangible outcomes of the MOSEM (Minds-On
experimental equipment kits in Superconductivity and ElectroMagnetism for the continuing vocational training of upper secondary school physics teachers—LLPLdV-TOI-2007-NO/165.009) project is the set of advanced experiments—High-Tech Kit (HTK). The Kit contains the experiments, prototyped and tested among the project partners’ schools and teacher training institutions. The activities are combined with e-modules comprising videos, animations, and modeling as well as with new support material for teachers and teacher seminars. The paper briefly shows some of the HTK materials as appropriate use of real and virtual multimedia in physics teaching and learning. The authors discuss the process of setting up same of the experiments and illustrate activities with the results of measurements obtained within
Transition Spectra for a BCS Superconductor with Multiple Gaps: Model Calculations for MgB_2
We analyze the qualitative features in the transition spectra of a model
superconductor with multiple energy gaps, using a simple extension of the
Mattis-Bardeen expression for probes with case I and case II coherence factors.
At temperature T = 0, the far infrared absorption edge is, as expected,
determined by the smallest gap. However, the large thermal background may mask
this edge at finite temperatures and instead the secondary absorption edges
found at Delta_i+Delta_j may become most prominent. At finite T, if certain
interband matrix elements are large, there may also be absorption peaks at the
gap difference frequencies | Delta_i-Delta_j | . We discuss the effect of
sample quality on the measured spectra and the possible relation of these
predictions to the recent infrared absorption measurement on MgB_2
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Thermal conductivity of MgB in the superconducting state
We present thermal conductivity measurements on very pure and dense bulk
samples, as indicated by residual resistivity values as low as 0.5 mW cm and
thermal conductivity values higher than 200 W/mK. In the normal state we found
that the Wiedemann Franz law, in its generalized form, works well suggesting
that phonons do not contribute to the heat transport. The thermal conductivity
in the superconducting state has been analysed by using a two-gap model. Thank
to the large gap anisotropy we were able to evaluate quantitatively intraband
scattering relaxation times of and bands, which depend on the
disorder in different way; namely, as the disorder increases, it reduces more
effectively the relaxation times of than of bands, as
suggested by a recent calculation [1].Comment: 12 pages, 5 figure
Nonmagnetic impurity effects in MgB
We study nonmagnetic impurity effects in MgB using the quasiclassical
equations of superconductivity for a weak-coupling two-band model. Parameters
in the model are fixed so as to reproduce experiments on MgB as closely
as possible. The quasiparticle density of states and the specific heat are
calculated for various values of the interband impurity scattering. The density
of states changes gradually from a two-gap structure into the conventional
single-gap structure as the interband scattering increases. It is found that
the excitation threshold is not a monotonic function of the interband
scattering. Calculated results for the specific heat are in good agreements
with experiments on samples after irradiation
Modeling multidimensional effects in the propagation of radiative shocks
Radiative shocks (also called supercritical shocks) are high Mach number shock waves that photoionize the medium ahead of the shock front and give rise to a radiative precursor. They are generated in the laboratory using high-energy or high-power lasers and are frequently present in a wide range of astronomical objects. Their modelisation in one dimension has been the subject of numerous studies, but generalization to three dimensions is not straightforward. We calculate analyticaly the absorption of radiation in a grey uniform cylinder and show how it decreases with , the product of the opacity and of the cylinder radius . Simple formulas, whose validity range increases when diminishes, are derived for the radiation field on the axis of symmetry. Numerical calculations in three dimensions of the radiative energy density, flux and pressure created by a stationary shock wave show how the radiation decreases whith . Finally, the bidimensional structures of both the precursor and the radiation field are calculated with time-dependent radiation hydrodynamics numerical simulations and the influence of two-dimensional effects on the electron density, the temperature, the shock velocity and the shock geometry are exhibited. These simulations show how the radiative precursor shortens, cools and slows down when is decreased
First-Principles Calculation of the Superconducting Transition in MgB2 within the Anisotropic Eliashberg Formalism
We present a study of the superconducting transition in MgB2 using the
ab-initio pseudopotential density functional method and the fully anisotropic
Eliashberg equation. Our study shows that the anisotropic Eliashberg equation,
constructed with ab-initio calculated momentum-dependent electron-phonon
interaction and anharmonic phonon frequencies, yields an average
electron-phonon coupling constant lambda = 0.61, a transition temperature Tc =
39 K, and a boron isotope-effect exponent alphaB = 0.31 with a reasonable
assumption of mu* = 0.12. The calculated values for Tc, lambda, and alphaB are
in excellent agreement with transport, specific heat, and isotope effect
measurements respectively. The individual values of the electron-phonon
coupling lambda(k,k') on the various pieces of the Fermi surface however vary
from 0.1 to 2.5. The observed Tc is a result of both the raising effect of
anisotropy in the electron-phonon couplings and the lowering effect of
anharmonicity in the relevant phonon modes.Comment: 4 pages, 3 figures, 1 tabl
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