Sharpening and characterizing of YBa₂Cu₃O7-δ probes for scanning probe microscopy

The realization of a scanning tunneling microscope with a high-temperature superconducting (HTS) probe could be a powerful tool for conveniently using liquid nitrogen to observe conductive molecules and elucidate HTS tunnel junctions. For these purposes, we fabricated and characterized HTS probes based on YBa 2 Cu 3 O 7-δ (YBCO). A focused Ga-ion beam system was employed for YBCO probe sharpening, achieving a tip radius of curvature of 44.3 nm. Because Ga-ion contamination was localized on the probe surface, the damaged surface was cleaned by Ar-ion milling for 40 min to produce pure probes. To test the superconductivity around the tip, a YBCO probe was immersed in liquid nitrogen and brought into contact with a gold-sputtered film on an MgO substrate. The resultant tip resistance was confirmed to drop sharply at approximately 80 K. To investigate the tunneling characteristics, the YBCO probe and thin film were installed in a scanning tunneling microscope with pumped liquid nitrogen tanks. A sharp increase in the tunneling current was observed from approximately 3 V, owing to the work function of the YBCO material.