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

Abstract

Point-contact Andreev reflection and magnetoresistance measurements were done using amorphous tungsten carbide (WC
x
) 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.

KeywordsAndreev oscillations–Normal–superconducting nanostructures–Granular superconductivity–Focused ion beam