320 research outputs found
Effect of Carbon-Doping in Bulk Superconducting MgB2 Samples
Bulk superconducting samples of MgB2 were prepared by solid state reaction of
stoichiometric quantities of Mg turnings and B in a sealed Ta cylinder at 890 C
for 2 hours. The as-synthesized MgB2 samples had a Tc of 39 K, as defined as
the onset of diamagnetism. The crystal symmetry was found to be hexagonal with
lattice parameters, a=3.0856 A, and c=3.5199 A, similar to the literature
values. To study the effect of carbon doping in MgB2, various C-containing
samples of x varying from 0 to 1.00 in MgB2-xCx were prepared. Magnetic
characterizations indicate that the Tc onset is same for pure and C-doped
samples for x = 0.05, and 0.10. However, the shielding signal decreased
monotonically with C content, apparently due to the presence of carbon on the
grain boundaries that isolates grains and prevents flow of supercurrents on the
perimeter.Comment: 10 pages, 3 figure
Superconducting magnesium diboride films on Silicon with Tc0 about 24K grown via vacuum annealing from stoichiometric precursors
Superconducting magnesium diboride films with Tc0 ~ 24 K and sharp transition
\~ 1 K were successfully prepared on silicon substrates by pulsed laser
deposition from a stoichiometric MgB2 target. Contrary to previous reports,
anneals at 630 degree and a background of 2x10^(-4) torr Ar/4%H2 were performed
without the requirement of Mg vapor or an Mg cap layer. This integration of
superconducting MgB2 films on silicon may thus prove enabling in
superconductor-semiconductor device applications. Images of surface morphology
and cross-section profiles by scanning electron microscopy (SEM) show that the
films have a uniform surface morphology and thickness. Energy dispersive
spectroscopy (EDS) reveals these films were contaminated with oxygen,
originating either from the growth environment or from sample exposure to air.
The oxygen contamination may account for the low Tc for those in-situ annealed
films, while the use of Si as the substrate does not result in a decrease in Tc
as compared to other substrates.Comment: 5 pages, 4 figures, 15 references; due to file size limit, images
were blure
Superconducting magnesium diboride films with Tc \approx 24K grown by pulsed laser deposition with in-situ anneal
Thin superconducting films of magnesium diboride (MgB2) with Tc \approx 24K
were prepared on various oxide substrates by pulsed laser deposition (PLD)
followed by an in-situ anneal. A systematic study of the influence of various
in-situ annealing parameters shows an optimum temperature of about 600C in a
background of 0.7 atm. of Ar/4%H2 for layers consisting of a mixture of
magnesium and boron. Contrary to ex-situ approaches (e.g. reacting boron films
with magnesium vapor at 900C), these films are processed below the
decomposition temperature of MgB2. This may prove enabling in the formation of
multilayers, junctions, and epitaxial films in future work. Issues related to
the improvement of these films and to the possible in-situ growth of MgB2 at
elevated temperature are discussed.Comment: 5 pages, 4 figure
Study of the microwave electrodynamic response of MgB2 thin films
We present a study on the power dependence of the microwave surface impedance
in thin films of the novel superconductor MgB2. 500 nm thick samples exhibiting
critical temperatures ranging between 26 and 38 K are synthesized by an ex-situ
post-anneal of e-beam evaporated boron in the presence of an Mg vapor at 900 C.
Preliminary results on films grown in situ by a high rate magnetron sputtering
technique from stoichiometric MgB2 and Mg targets are also reported. Microwave
measurements have been carried out employing a dielectrically loaded niobium
superconducting cavity operating at 19.8 GHz and 4 K. The study shows that the
electrodynamic response of MgB2 films is presently dominated by extrinsic
sources of dissipation, appearing already at low microwave power, likely to be
ascribed to the presence of grain boundaries and normal inclusions in the
samples.Comment: 10 pages, 3 figures, proceedings of the conference EUCAS 2001,
submitted to Physica
High Critical Current Coated Conductors
One of the important critical needs that came out of the DOE’s coated conductor workshop was to develop a high throughput and economic deposition process for YBCO. Metal-organic chemical vapor deposition (MOCVD) technique, the most critical steps in high technical micro fabrications, has been widely employed in semiconductor industry for various thin film growth. SuperPower has demonstrated that (Y,Gd)BCO films can be deposited rapid with world record performance. In addition to high critical current density with increased film thickness, flux pinning properties of REBCO films needs to be improved to meet the DOE requirements for various electric-power equipments. We have shown that doping with Zr can result in BZO nanocolumns, but at substantially reduced deposition rate. The primary purpose of this subtask is to develop high current density MOCVD-REBCO coated conductors based on the ion-beam assisted (IBAD)-MgO deposition process. Another purpose of this subtask is to investigate HTS conductor design optimization (maximize Je) with emphasis on stability and protection issues, and ac loss for REBCO coated conductors
Soft x-ray spectroscopy measurements of the p-like density of states of B in MgB2 and evidence for surface boron oxides on exposed surfaces
Soft X-ray absorption and fluorescence measurements are reported for the
K-edge of B in MgB2. The measurements confirm a high density of B
pxy(sigma)-states at the Fermi edge and extending to approximately 0.9 eV above
the edge. A strong resonance is observed in elastic scattering through a
core-exciton derived from out-of-plane pz(pi*)-states. Another strong
resonance, observed in both elastic and inelastic spectra, is identified as a
product of surface boron oxides.Comment: 7 pages total, 4 figures, submitted to Phys. Rev. Let
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