The effect of magnetic field on the two superconducting gaps in MgB2

TL;DR

This study investigates how magnetic fields affect the two superconducting gaps in MgB2, revealing different behaviors depending on field orientation and providing insights into pair breaking and gap suppression.

Contribution

The paper presents detailed measurements of the magnetic field dependence of the two superconducting gaps in MgB2, highlighting anisotropic effects and the role of pair breaking.

Findings

Pi-band gap disappears around 1 T due to pair breaking

Order parameters survive up to 6.5 T in perpendicular field

Delta_sigma decreases gradually in parallel field, with higher Hc2

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 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.