Superconducting gap structure and pinning in disorderedMgB2 films

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 andmagnetic fields was evaluated from magnetization curves, the structure of thesuperconducting order parameter was obtained from point-contact spectroscopyand the scattering rates were evaluated by fitting the temperature dependentnormal-state resistivity to the two-band model. The films have similar criticaltemperatures close to 39 K, but the upper critical fields were different by afactor of 2 (5.2 and 2.5 T at 20 K). We have found that the film with higherHc2 also had strongerscattering in the σ band and a smaller value of the superconducting gap in this band. As the scattering in theσ band is primarily due to the defects in the boron plane, our results are consistentwith the assumption that disordering the boron planes leads to enhancedHc2 and better pinning properties in a magnetic field.