5 research outputs found
Electron diffusivities in MgB2 from point contact spectroscopy
We demonstrate that the variation of the Andreev reflection with applied
magnetic field provides a direct means of comparing the properties of MgB2 with
the theory for a dirty two-band superconductor, and we find good agreement
between the two. The ratio of electron diffusivities in the s and p bands can
be inferred from this experiment. We find that the field dependence of the
density of states at the Fermi level in the p band is independent of the field
direction, and that the anisotropic upper critical field is determined by the
anisotropic diffusivity in the s band
The effect of magnetic field on the two superconducting gaps in MgB2
Double-gap superconductivity in an epitaxial MgB2 film has been studied by
means of point-contact spectroscopy in magnetic field up to 8 Tesla. The
relatively fast disappearance of the feature associated with the pi-band gap at
a field around 1 T is caused by the line broadening due to strong pair breaking
rather than to a collapse of the double-gap state. This pair breaking was found
to increase linearly with field. Field dependences of the order parameters
Delta_pi and Delta_sigma in the pi and sigma bands were measured in field
applied parallel and perpendicular to the film, at T = 4.2 K. In perpendicular
field, both order parameters survive to a common Hc2, which is about 6.5 T for
this direction. In parallel field, the decrease of Delta_sigma is much more
gradual, consistent with the Hc2 being about 4 times greater in this
ori-entation. The difference in Delta_pi measured in the two field orientations
is however smaller than the difference of Delta_sigma. We compare these results
with the data on tunnelling spectroscopy and specific heat measurements of MgB2
single crystals and find consistency between the different experimental
approaches.Comment: 4 pages, 5 figures Revised: discussion of the distribution of gap
value
Superconducting gap structure and pinning in disordered MgB2 films
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 magnetisation curves, the
structure of 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 (5.2T and 2.5 T at 20 K). We have found that
the film with higher Hc2 also had stronger scattering in the sigma band and
smaller value of the superconducting gap in this band. As the scattering in
sigma band is primarily due to the defects in boron plane, our results are
consistent with the assumption that disordering the boron planes leads to
enhanced Hc2 and better pinning properties in magnetic field.Comment: Paper presented at EUCAS'0
Point-Contact Spectroscopy in MgB_2: from Fundamental Physics to Thin-Film Characterization
In this paper we highlight the advantages of using point-contact spectroscopy
(PCS) in multigap superconductors like MgB_2, both as a fundamental research
tool and as a non-destructive diagnostic technique for the optimization of
thin-film characteristics. We first present some results of crucial fundamental
interest obtained by directional PCS in MgB_2 single crystals, for example the
temperature dependence of the gaps and of the critical fields and the effect of
a magnetic field on the gap amplitudes. Then, we show how PCS can provide
useful information about the surface properties of MgB_2 thin films (e.g. Tc,
gap amplitude(s), clean or dirty-limit conditions) in view of their
optimization for the fabrication of tunnel and Josephson junctions for
applications in superconducting electronics.Comment: 11 pages, 7 figures; Proceedings of 6th EUCAS Conference (14-18
September 2003, Sorrento - Italy