Andreev Reflection and Granular Superconductivity Features Observed in Mesoscopic Samples Using Amorphous Tungsten Carbide Superconductors

Point-contact Andreev reflection and magnetoresistance measurements were done using amorphous tungsten carbide (WCx) superconductors. Superconducting tips as well as microwires were grown directly under a focused Ga+-ion beam (FIB) on pre-patterned samples. Using e-beam lithography, the electrical contacts were prepared later using a special geometry. Current–voltage measurements as a function of temperature and magnetic field clearly showed the signatures of Andreev reflection. We observed anomalies in the differential conductivity at voltages above the energy gap values. These anomalies can be well understood as due to a weak-link formation with narrow band properties contributing in parallel at the interfaces of the contacts. We also observed Andreev oscillations as a function of magnetic field similar to those found recently in specially prepared normal–superconducting–normal nanostructures and multigraphene samples. In some of the structures, we were able to produce interfaces in which clear granular behavior in a certain temperature region was observed, such as, for example, an anomalous field hysteresis loop compatible with the existence of granular superconductivity, similar to that found also in thin mesoscopic graphite samples.