The effect of magnetic field on the two superconducting gaps in MgB2
Abstract
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 Deltapi and Deltasigma 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 Deltasigma is much more gradual, consistent with the Hc2 being about 4 times greater in this ori-entation. The difference in Deltapi measured in the two field orientations is however smaller than the difference of Deltasigma. 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.
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