Superconducting gap structure and pinning in disordered MgB2 films

TL;DR

This study compares two MgB2 thin films, revealing how disorder in boron planes enhances upper critical fields and pinning by increasing scattering in the sigma band, despite similar critical temperatures.

Contribution

It provides experimental evidence linking boron plane disorder to increased Hc2 and improved vortex pinning in MgB2 films.

Findings

Higher Hc2 correlates with stronger sigma band scattering.

Disordering boron planes enhances pinning properties.

Superconducting gap in sigma band decreases with increased scattering.

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 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.