51 research outputs found
Vortex fluctuations in superconducting La-Sr-Cu-O
Vortex fluctuations in the system have been
studied as a function of magnetic field, temperature and carrier concentration
in order to determine the dimensionality of the fluctuations. For a
sample, there is a unique crossing-temperature on the magnetization vs.
temperature plots for all magnetic fields up to 7 T, and the data scale very
well with 2D fluctuation theory. At lower x-values where is much
smaller, there are two well defined crossing points, one at low fields
(typically less than 1 T) and another at high fields (typically 3-7 T). A fit
of the data to fluctuation theory shows that the low field crossing data scale
as 2D fluctuations and the high field crossing data scale as 3D fluctuations.
It would appear that as the magnetic field approaches , there is a 2D
to 3D cross-over where the low field 2D pancake vortex structure transforms
into a 3D vortex structure
Microstructure and pinning properties of hexagonal-disc shaped single crystalline MgB2
We synthesized hexagonal-disc-shaped MgB2 single crystals under high-pressure
conditions and analyzed the microstructure and pinning properties. The lattice
constants and the Laue pattern of the crystals from X-ray micro-diffraction
showed the crystal symmetry of MgB2. A thorough crystallographic mapping within
a single crystal showed that the edge and c-axis of hexagonal-disc shape
exactly matched the (10-10) and the (0001) directions of the MgB2 phase. Thus,
these well-shaped single crystals may be the best candidates for studying the
direction dependences of the physical properties. The magnetization curve and
the magnetic hysteresis for these single crystals showed the existence of a
wide reversible region and weak pinning properties, which supported our single
crystals being very clean.Comment: 5 pages, 3 figures. submitted to Phys. Rev.
Quasiclassical Approach to Transport in the Vortex State and the Hall Effect
We derive generalized quasiclassical transport equations which include the
terms responsible for the Hall Effect in the vortex state of a clean type-II
superconductor, and calculate the conductivity tensor for an s-wave
superconductor in the high-field regime. We find that below the superconducting
transition the contribution to the transverse conductivity due to dynamical
fluctuations of the order parameter is compensated by the modification of the
quasiparticle contribution. In this regime the nonlinear behaviour of the Hall
angle is governed by the change in the effective quasiparticle scattering rate
due to the reduction in the density of states at the Fermi level. The
connection with experimental results is discussed.Comment: 32 pages, 3 postscript figure
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
A Magnetization and Microwave Study of Superconducting MgB2
We report magnetic field dependent magnetization and microwave impedance
measurements on a MgB2 superconductor prepared by high pressure synthesis. We
find that the upper critical field is linearly dependent on temperature near Tc
and the dc irreversibility field exponent is ~1.4. The microwave data display
an excess surface resistance below Tc which is neither observed in low Tc nor
in high temperature superconductors (HTSC). The real part of the complex
conductivity, sigma1, shows a huge maximum below Tc and the imaginary part,
sigma2, is linear for temperatures less than 20 K, which can not be simply
accounted for by the weak coupling BCS model with an s-wave superconducting
order parameter. We speculate that this may be due to the two gaps reported by
other studies. Unlike measurements on the high temperature superconducting
cuprates, we find no evidence of weak-links in the superconducting state. By
inverting the magnetic field dependent impedance data, we find a vortex
depinning frequency that decreases with increasing magnetic field and evidence
for an anisotropic upper critical magnetic field.Comment: Submitted to Phys. Rev. B. 19 pages PD
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