141 research outputs found
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
Limits of the upper critical field in dirty two-gap superconductors
An overview of the theory of the upper critical field in dirty two-gap
superconductors, with a particular emphasis on MgB is given. We focus here
on the maximum which may be achieved by increasing intraband
scattering, and on the limitations imposed by weak interband scattering and
paramagnetic effects. In particular, we discuss recent experiments which have
recently demonstrated ten-fold increase of in dirty carbon-doped films
as compared to single crystals, so that the parallel to the ab
planes may approach the BCS paramagnetic limit, . New effects produced by weak interband scattering in the two-gap
Ginzburg-Landau equations and in ultrathin MgB films are
addressed.Comment: Submitted to the special issue of Physica C on MgB on Dec. 3,
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Manganese nodule geochemistry from the Challenger Expedition
C. W. Gümbel received nodules from J. Murray which were collected at a depth of 2740 fathoms, between Japan and the Sandwich Islands, by the "Challenger" Expedition. They were either round or long in shape, with a dull, dirty-brown coloured surface, and enclosed fragments of pumice-stone, and more rarely teeth of sharks or fragments of mussels. They were analysed by A. Schwager
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