16 research outputs found
Improved Hc2 in Bulk-Form Magnesium Diboride by Mechanical Alloying With Carbon
High energy milling of MgB2 pre-reacted powder renders the material largely
amorphous through extreme mechanical deformation and is suitable for
mechanically alloying MgB2 with dopants including carbon. Bulk samples of
milled C and MgB2 powders subjected to hot isostatic pressing and Mg vapor
annealing have achieved critical fields in excess of 32T and critical current
density approaching 10^6 A/cm^2.Comment: 13 pages, 3 figures references updated, typos corrected, numerical
error correcte
Synthesis and Properties of c-axis Oriented Epitaxial MgB2 Thin Films
We report the growth and properties of epitaxial MgB2 thin films on (0001)
Al2O3 substrates. The MgB2 thin films were prepared by depositing boron films
via RF magnetron sputtering, followed by a post-deposition anneal at 850C in
magnesium vapor. X-ray diffraction and cross-sectional TEM reveal that the
epitaxial MgB2 films are oriented with their c-axis normal to the (0001) Al2O3
substrate and a 30 degree rotation in the ab-plane with respect to the
substrate. The critical temperature was found to be 35 K and the anisotropy
ratio, Hc2(parallel to the film) / Hc2(pendicular to the film), about 3 at 25K.
The critical current densities at 4.2 K and 20 K (at 1 T perpendicular magnetic
field) are 5x10E6 A/cm2 and 1x10E6 A/cm2, respectively. The controlled growth
of epitaxial MgB2 thin films opens a new avenue in both understanding
superconductivity in MgB2 and technological applications.Comment: 10 pages, 6 figure
Very high upper critical fields in MgB2 produced by selective tuning of impurity scattering
We report a significant enhancement of the upper critical field of
different samples alloyed with nonmagnetic impurities. By studying
films and bulk polycrystals with different resistivities , we show a
clear trend of increase as increases. One particular high
resistivity film had zero-temperature well above the
values of competing non-cuprate superconductors such as and Nb-Ti. Our
high-field transport measurements give record values and for high resistivity films and
for untextured bulk polycrystals. The highest
film also exhibits a significant upward curvature of , and
temperature dependence of the anisotropy parameter opposite to that of single crystals: decreases as the
temperature decreases, from to .
This remarkable enhancement and its anomalous temperature dependence
are a consequence of the two-gap superconductivity in , which offers
special opportunities for further increase by tuning of the impurity
scattering by selective alloying on Mg and B sites. Our experimental results
can be explained by a theory of two-gap superconductivity in the dirty limit.
The very high values of observed suggest that can be made
into a versatile, competitive high-field superconductor.Comment: An updated version of the paper (12/12/2002)that was placed on
cond-mat on May 7 200
High-field superconductivity in alloyed MgB2 thin films
We investigated the effect of alloying on the upper critical field
in 12 films, in which disorder was introduced by growth, carbon doping
or He-ion irradiation, finding a significant enhancement in C-alloyed
films, and an anomalous upward curvature of . Record high values of
and were observed
perpendicular and parallel to the ab plane, respectively. The temperature
dependence of is described well by a theory of dirty two-gap
superconductivity. Extrapolation of the experimental data to T=0 suggests that
approaches the paramagnetic limit of
Properties of MgB2 thin films with carbon doping
We have studied structural and superconducting properties of MgB2 thin films doped with carbon during the hybrid physical-chemical vapor deposition process. A carbon-containing precursor metalorganic bis(methylcyclopentadienyl)magnesium was added to the carrier gas to achieve carbon doping. As the amount of carbon in the film increases, the resistivity increases, T-c decreases, and the upper critical field increases dramatically as compared to clean films. The self-field J(c) in the carbon doped film is lower than that in the clean film, but J(c) remains relatively high to much higher magnetic fields, indicating stronger pinning. Structurally, the doped films are textured with columnar nano-grains and highly resistive amorphous areas at the grain boundaries. The carbon doping approach can be used to produce MgB2 materials for high magnetic-field applications. (C) 2004 American Institute of Physics
MgB2 energy gap determination by scanning tunnelling spectroscopy
We report scanning tunnelling spectroscopy (STS) measurements of the gap properties of both ceramic MgB2 and c-axis oriented epitaxial MgB2 thin films. Both show a temperature dependent zero bias conductance peak and evidence for two superconducting gaps. We report tunnelling spectroscopy of superconductor-insulator-superconductor (S-I-S) junctions formed in two ways in addition to normal metal-insulator-superconductor (N-I-S) junctions. We find a gap Δ = 2.2-2.8 meV, with spectral features and temperature dependence that are consistent between S-I-S junction types. In addition, we observe evidence of a second, larger gap, Δ = 7.2 meV, consistent with a proposed two-hand model