Electrical Transport Study of YBCO Thin Films at Millikelvin Temperatures

In this manuscript, we explore tunnel spectroscopy of insulating barrier YBCO tunnel junctions fabricated with focused helium ion irradiation. Five junctions with different tunneling conductance were measured in a dilution refrigerator at temperatures between 50 and 800 millikelvin. The differential conductance of all devices exhibited an oscillation that was periodic with bias current with a period of about one micro amp. This feature was reproduced in all devices tested. We attribute this behavior to resonant tunneling sates above the energy gap.