  We report on a controlled method to fabricate in-situ a superconducting (SC)
nanostructure at the apex of the standard W tip of a Scanning Tunnelling
Microscope (STM) by pulsing the tunnelling voltage on a graphene covered metal
surface. We have characterized the SC properties of the resulting tip as a
function of temperature and magnetic field, obtaining a transition temperature
of 3.3 K and a critical field well above 3T. The SC nanotip is robust, stable,
and achieves atomic resolution. The non-SC tip can be easily recovered by
controlled voltage pulsing on a clean metal surface. The present result should
be taken into account when studying zero-bias features like Kondo resonances or
superconductivity on graphene-based systems by means of STM using tungsten
tips.
