1,390 research outputs found
Conductance asymmetry in point-contacts on epitaxial thin films of Ba(FeCo)As
Point-contact spectroscopy is a powerful tool for probing superconductors.
One of the most common observations in the point-contact spectra on the
recently discovered ferropnictide superconductors is a large conductance
asymmetry with respect to voltage across the point-contact. In this paper we
show that the antisymmetric part of the point-contact spectrum between a silver
tip and an epitaxial thin film of Ba(FeCo)As shows
certain unique features. These features have an interesting evolution with
increasing temperature up to a temperature that is 30% larger than the critical
temperature of the superconductor. We argue that this evolution can be
associated with the rich normal state properties of these materials.Comment: 4 pages, 2 figure
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
A Trapped Field of 17.6 T in Melt-Processed, Bulk Gd-Ba-Cu-O Reinforced with Shrink-Fit Steel
The ability of large grain, REBaCuO [(RE)BCO; RE =
rare earth] bulk superconductors to trap magnetic field is determined by their
critical current. With high trapped fields, however, bulk samples are subject
to a relatively large Lorentz force, and their performance is limited primarily
by their tensile strength. Consequently, sample reinforcement is the key to
performance improvement in these technologically important materials. In this
work, we report a trapped field of 17.6 T, the largest reported to date, in a
stack of two, silver-doped GdBCO superconducting bulk samples, each of diameter
25 mm, fabricated by top-seeded melt growth (TSMG) and reinforced with
shrink-fit stainless steel. This sample preparation technique has the advantage
of being relatively straightforward and inexpensive to implement and offers the
prospect of easy access to portable, high magnetic fields without any
requirement for a sustaining current source.Comment: Updated submission to reflect licence change to CC-BY. This is the
"author accepted manuscript" and is identical in content to the published
versio
Electrodynamics of superconducting pnictide superlattices
It has been recently reported (S. Lee et al., Nature Materials 12, 392, 2013)
that superlattices where layers of the 8% Co-doped BaFe2As2 superconducting
pnictide are intercalated with non superconducting ultrathin layers of either
SrTiO3 or of oxygen-rich BaFe2As2, can be used to control flux pinning, thereby
increasing critical fields and currents, without significantly affecting the
critical temperature of the pristine superconducting material. However, little
is known about the electron properties of these systems. Here we investigate
the electrodynamics of these superconducting pnictide superlattices in the
normal and superconducting state by using infrared reflectivity, from THz to
visible range. We find that multi-gap structure of these superlattices is
preserved, whereas some significant changes are observed in their electronic
structure with respect to those of the original pnictide. Our results suggest
that possible attempts to further increase the flux pinning may lead to a
breakdown of the pnictide superconducting properties.Comment: 4 pages, two figure
Atmospheric conditions and their effect on ball-milled magnesium diboride
Magnesium diboride bulk pellets were fabricated from pre-reacted MgB2 powder
ball milled with different amounts of exposure to air. Evidence of increased
electron scattering including increased resistivity, depressed Tc, and enhanced
Hc2 of the milled and heat treated samples were observed as a result of
increased contact with air. These and other data were consistent with alloying
with carbon as a result of exposure to air. A less clear trend of decreased
connectivity associated with air exposure was also observed. In making the case
that exposure to air should be considered a doping process, these results may
explain the wide varibability of "undoped" MgB2 properties extant in the
literature.Comment: Work presented at ASC 2006 in Seattl
Nanoscale grains, high irreversibility field, and large critical current density as a function of high energy ball milling time in C-doped magnesium diboride
Magnesium diboride (MgB2) powder was mechanically alloyed by high energy ball
milling with C to a composition of Mg(B0.95C0.05)2 and then sintered at 1000 C
in a hot isostatic press. Milling times varied from 1 minute to 3000 minutes.
Full C incorporation required only 30-60 min of milling. Grain size of sintered
samples decreased with increased milling time to less than 30 nm for 20-50 hrs
of milling. Milling had a weak detrimental effect on connectivity. Strong
irreversibility field (H*) increase (from 13.3 T to 17.2 T at 4.2 K) due to
increased milling time was observed and correlated linearly with inverse grain
size (1/d). As a result, high field Jc benefited greatly from lengthy powder
milling. Jc(8 T, 4.2 K) peaked at > 80,000 A/cm2 with 1200 min of milling
compared with only ~ 26,000 A/cm2 for 60 min of milling. This non-compositional
performance increase is attributed to grain refinement of the unsintered powder
by milling, and to the probable suppression of grain growth by milling-induced
MgO nano-dispersions.Comment: 12 pages, 11 figure
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