1,990 research outputs found
Anisotropic properties of MgB2 by torque magnetometry
Anisotropic properties of superconducting MgB2 obtained by torque
magnetometry are compared to theoretical predictions, concentrating on two
issues. Firstly, the angular dependence of Hc2 is shown to deviate close to Tc
from the dependence assumed by anisotropic Ginzburg-Landau theory. Secondly,
from the evaluation of torque vs angle curves it is concluded that the
anisotropy of the penetration depth gamma_lambda has to be substantially higher
at low temperature than theoretical estimates, at least in fields higher than
0.2 T.Comment: 2 p.,2 Fig., submitted to Physica C (M2S-Rio proceedings); v2: 1 ref
adde
Anisotropy of the superconducting state properties and phase diagram of MgB2 by torque magnetometry on single crystals
The angular and temperature dependence of the upper critical field Hc2 in
MgB2 was determined from torque magnetometry measurements on single crystals.
The Hc2 anisotropy gamma_H was found to decrease with increasing temperature,
in disagreement with the anisotropic Ginzburg-Landau theory, which predicts
that the gamma_H is temperature independent. This behaviour can be explained by
the two band nature of superconductivity in MgB2. An analysis of measurements
of the reversible torque in the mixed state yields a field dependent effective
anisotropy gamma_eff, which can be at least partially explained by different
anisotropies of the penetration depth and the upper critical field. It is shown
that a peak effect in fields of about 0.85 Hc2 is a manifestation of an
order-disorder phase transition of vortex matter. The H-T phase diagram of MgB2
for H//c correlates with the intermediate strength of thermal fluctuations in
MgB2, as compared to those in high and low Tc superconductors.Comment: 12 pages, 8 figures, Physica C in print (invited paper for a special
issue on MgB2
A small sealed Ta crucible for thermal analysis of volatile metallic samples
Differential thermal analysis on metallic alloys containing volatile elements
can be highly problematic. Here we show how measurements can be performed in
commercial, small-sample, equipment without modification. This is achieved by
using a sealed Ta crucible, easily fabricated from Ta tubing and sealed in a
standard arc furnace. The crucible performance is demonstrated by measurements
on a mixture of Mg and MgB, after heating up to 1470. We
also show data, measured on an alloy with composition GdMg, that
clearly shows both the liquidus and a peritectic, and is consistent with
published phase diagram data
Dielectric properties of charge ordered LuFe2O4 revisited: The apparent influence of contacts
We show results of broadband dielectric measurements on the charge ordered,
proposed to be mul- tiferroic material LuFe2O4. The temperature and frequency
dependence of the complex permittivity as investigated for temperatures above
and below the charge-oder transition near T_CO ~ 320 K and for frequencies up
to 1 GHz can be well described by a standard equivalent-circuit model
considering Maxwell-Wagner-type contacts and hopping induced AC-conductivity.
No pronounced contribution of intrinsic dipolar polarization could be found and
thus the ferroelectric character of the charge order in LuFe2O4 has to be
questioned.Comment: 4 pages, 3 figure
Point-contact spectroscopy of Al- and C-doped MgB2. Superconducting energy gaps and scattering studies
The two-band/two-gap superconductivity in aluminium and carbon doped MgB
has been addressed by the point-contact spectroscopy. Two gaps are preserved in
all samples with down to 22 K. The evolution of two gaps as a function
of the critical temperature in the doped systems suggest the dominance of the
band-filling effects but for the increased Al-doping the enhanced interband
scattering approaching two gaps must be considered. The magnetic field
dependences of the Andreev reflection excess currents as well as zero-energy
density of states determined from the experimental data are used to analyze the
intraband scattering. It is shown, that while the C-doping increases the
intraband scattering in the -band more rapidly then in the band,
the Al-doping does not change their relative weight.Comment: 8 pages, incl. 6 figure
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