Spatial variations of the superconductor gap structure in MgB2/Al composite

Composites consisting of MgB2 and Al, 11% by volume, undergo a transition-to-superconductivity onset at TC = 38 K, close to the value reported for pure MgB2. The transition appears to have broadened, as determined by both magnetic and transport measurements, possibly due to the proximity effect and disorder. Spatially resolved tunnelling spectroscopy at 4.2 K shows a distribution of gap structures, from Bardeen-Cooper-Schrieffer-like spectra with 2Δ/kBTC = 3.2 to spectra that are typical for proximity superconductors.