Fabrication and characterization of scanning tunneling microscopy superconducting Nb tips having highly enhanced critical fields

摘要

We report a simple method for the fabrication of Niobium. superconducting (SC) tips for scanning tunneling microscopy which allow atomic resolution. The tips, formed in situ by the mechanical breaking of a niobium wire, reveal a clear SC gap of 1.5 meV and a critical temperature T-c = 9.2 +/- 0.3 K, as deduced from Superconductor Insulator Normal metal (SIN) and Superconductor Insulator Superconductor (SIS) spectra. These match the values of bulk Nb samples. We systematically find an enhanced value of the critical magnetic field in which superconductivity in the tip is destroyed (around 1T for some tips) up to five times larger than the critical field of bulk Nb (0.21 T). Such enhancement is attributed to a size effect at the tip apex. (C) 2004 Elsevier B.V. All rights reserved.