5 research outputs found
Superconducting properties of MgB2 thin films prepared on flexible plastic substrates
Superconducting MgB2 thin films were prepared on 50-micrometer-thick,
flexible polyamide Kapton-E foils by vacuum co-deposition of Mg and B
precursors with nominal thickness of about 100 nm and a special ex-situ rapid
annealing process in an Ar or vacuum atmosphere. In the optimal annealing
process, the Mg-B films were heated to approximately 600 C, but at the same
time, the backside of the structures was attached to a water-cooled radiator to
avoid overheating of the plastic substrate. The resulting MgB2 films were
amorphous with the onset of the superconducting transition at T_(c,on) about 33
K and the transition width of approximately 3 K. The critical current density
was > 7x10^5 A/cm^2 at 4.2 K, and its temperature dependence indicated a
granular film composition with a network of intergranular weak links. The films
could be deposited on large-area foils (up to 400 cm^2) and, after processing,
cut into any shapes (e.g., stripes) with scissors or bent multiple times,
without any observed degradation of their superconducting properties.Comment: 3 figure
Phonons in MgB2 by Polarized Raman Scattering on Single Crystals
The paper presents detailed Raman scattering study of the unusually broad E2g
phonon mode in MgB2 crystal. For the first time, it is shown by the polarized
Raman scattering on few-micron-size crystallites with natural faces that the
observed broad Raman feature really does obey the selection rules of an E2g
mode. Raman spectra on high quality polycrystalline superconducting MgB2 wires
reveal a very symmetric E2g phonon line near 615 1/cm with the room temperature
linewidth of 260 1/cm only. Additional scattering of different polarization
dependence, observed in certain crystallites is interpreted as weighted phonon
density of states induced by lattice imperfections.Comment: 4 pages + 7 figure
Time-resolved dynamics of the superconducting two-gap state in MgB2 thin films
Femtosecond pump-probe studies show that carrier dynamics in MgB2 films is
governed by the sub-ps electron-phonon (e-ph) relaxation present at all
temperatures, the few-ps e-ph process well pronounced below 70 K, and the
sub-ns superconducting relaxation below Tc. The amplitude of the
superconducting component versus temperature follows the superposition of the
isotropic dirty gap and the 3-dimensional gap dependences, closing at two
different Tc values. The time constant of the few-ps relaxation exhibits a
double-divergence at temperatures corresponding to the Tc's of the two gaps.Comment: Accepted for publication in Phys.Rev.Let
In situ annealing of superconducting MgB2 films prepared by pulsed laser deposition
The in situ annealing conditions of pulsed laser deposited MgB2 films were
studied. The precursor films were deposited at 250 C from a stoichiometric MgB2
target in a 120mTorr Ar atmosphere. The films were then in situ annealed at a
temperature from 450 C to 800 C and an annealing time from 1 minute to 10
minutes. We found that the superconducting properties depend in a crucial way
on the annealing conditions: temperature, heating rate and time. The best film
with a thickness of ~600nm was obtained under the following annealing
conditions: Tanneal=680-690 C, tanneal=1 min, heating rate= 38 C/min. The Tc
onset of the film is 28K with a transition width of ~10K. The hysteresis loop
of magnetic moment of the film indicates weak field dependence in high fields.
Magneto-optical imaging of the film showed quite homogeneous magnetic flux
penetration, indicating structural homogeneity. The films without annealing
showed no superconductivity.Comment: 12 pages, 6 figure