378 research outputs found
Au Fe vs Cu thermocouples
A calibration of gold iron thermocouples is given
Superconductivity in Dense Wires
becomes superconducting just below 40 K. Whereas porous
polycrystalline samples of can be synthesized from boron powders, in
this letter we demonstrate that dense wires of can be prepared by
exposing boron filaments to vapor. The resulting wires have a diameter of
160 , are better than 80% dense and manifest the full shielding in the superconducting state. Temperature-dependent
resistivity measurements indicate that is a highly conducting metal in
the normal state with = 0.38 -. Using this value, an
electronic mean free path, can be estimated, indicating
that wires are well within the clean limit. , , and
data indicate that manifests comparable or better superconducting
properties in dense wire form than it manifests as a sintered pellet.Comment: Figures' layout fixe
Superconducting MgB2 thin films by pulsed laser deposition
Growth of MgB2 thin films by pulsed laser deposition is examined under ex
situ and in situ processing conditions. For the ex situ process, Boron films
grown by PLD were annealed at 900 C with excess Mg. For the in situ process,
different approaches involving ablation from a stoichiometric target under
different growth conditions, as well as multilayer deposition involving
interposed Mg layers were examined and analyzed. Magnetic measurements on ex
situ processed films show TC of ~39 K, while the current best in situ films
show a susceptibility transition at ~ 22 K.Comment: 3 pages, PD
Method of increasing magnetostrictive response of rare earth-iron alloy rods
This invention comprises a method of increasing the magnetostrictive response of rare earth iron (RFe) magnetostrictive alloy rods by a thermal-magnetic treatment. The rod is heated to a temperature above its Curie temperature, viz. from 400 rod is at that temperature, a magnetic field is directionally applied and maintained while the rod is cooled, at least below its Curie temperature
Phonon dispersion and electron-phonon coupling in MgB_2 and AlB_2
We present a first principles investigation of the lattice dynamics and
electron-phonon coupling of the superconductor MgB_2 and the isostructural
AlB_2 within the framework of density functional perturbation theory using a
mixed-basis pseudopotential method. Complete phonon dispersion curves and
Eliashberg functions \alpha^2F are calculated for both systems. We also report
on Raman measurements, which support the theoretical findings. The calculated
generalized density-of-states for MgB_2 is in excellent agreement with recent
neutron-scattering experiments. The main differences in the calculated phonon
spectra and \alpha^2F are related to high frequency in-plane boron vibrations.
As compared to AlB_2, they are strongly softened in MgB_2 and exhibit an
exceptionally strong coupling to electronic states at the Fermi energy. The
total coupling constants are \lambda_{MgB_2}=0.73 and \lambda_{AlB_2}=0.43.
Implications for the superconducting transition temperature are briefly
discussed.Comment: 10 pages, 4 figures, to appear in Phys. Rev. Let
Systematic effects of carbon doping on the superconducting properties of Mg(BC)
The upper critical field, , of Mg(BC) has been
measured in order to probe the maximum magnetic field range for
superconductivity that can be attained by C doping. Carbon doped boron
filaments are prepared by CVD techniques, and then these fibers are then
exposed to Mg vapor to form the superconducting compound. The transition
temperatures are depressed about C and rises at about C. This means that 3.5% C will depress from to and
raise from to . Higher fields are probably
attainable in the region of 5% C to 7% C. These rises in are
accompanied by a rise in resistivity at from about
to about . Given that the samples are polycrystalline wire
segments, the experimentally determined curves represent the upper
manifold associated with
Magnetoresistivity and Complete in
Detailed magneto-transport data on dense wires of are reported for
applied magnetic fields up to 18 T. The temperature and field dependencies of
the electrical resistivity are consistent with behaving like a simple
metal and following a generalized form of Kohler's rule. In addition, given the
generally high values and narrow resistive transition widths associated
with synthesized in this manner, combined with applied magnetic fields
of up to 18 T, an accurate and complete curve could be determined.
This curve agrees well with curves determined from lower field measurements on
sintered pellets and wires of . is linear in over a wide
range of temperature (7 K 32 K) and has an upward curvature for
close to . These features are similar to other high , clean limit,
boron-bearing intermetallics: and .Comment: minor changes in styl
Penetration Depth and Anisotropy in MgB2
The penetration depth lambda of MgB2 was deduced from both the ac
susceptibility chi and the magnetization M(H) of sorted powders. The good
agreement between the two sets of data without geometric correction for the
grain orientation suggests that MgB2 is an isotropic superconductor.Comment: 9 pages, 5 figures; submitted to Physical Review B (February 28,
2001; revised June 28, 2001); reference list update
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