Superconductor-Insulator Transition in Ultrasmall Single Tunnel Junctions

We measured the temperature dependence of the resistance of ultrasmall Al single tunnel junctions. NiCr thin films or multiple junctions were used to fabricate measuring leads with lead resistance higher than the resistance quantum R K = h / e 2 =25.8 kΩ. We observed the superconductor-insulator (SI) transition in single tunnel junctions for the first time. The critical normal-state resistance of the SI transition is determined to be about 11 kΩ. The use of high-resistance leads was indispensable, since a low lead resistance prevents observation of the intrinsic nature of single junctions at low temperatures. We claim that this SI transition is a manifestation of the dissipative phase transition which is predicted to occur in Josephson junctions with strong dissipation.