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Abstract:
We have performed a comparative study of two thin films of magnesium
diboride (MgB2) grown by different techniques. The critical current
density at different temperatures and magnetic fields was evaluated
from magnetization curves, the structure of the superconducting order
parameter was obtained from point-contact spectroscopy and the
scattering rates were evaluated by fitting the temperature dependent
normal-state resistivity to the two-band model. The films have similar
critical temperatures close to 39 K, but the upper critical fields were
different by a factor of 2 (5.2 and 2.5 T at 20 K). We have found that
the film with higher H-c2 also had stronger scattering in the sigma
band and a smaller value of the superconducting gap in this band. As
the scattering in the a band is primarily due to the defects in the
boron plane, our results are consistent with the assumption that
disordering the boron planes leads to enhanced H-c2 and better pinning
properties in a magnetic field.