We have studied the transition from the tunneling regime to point contact with a low-temperature scanning tunneling microscope, when one of the electrodes is a normal metal (Au, Pt-Rh) and the other is a superconductor (Pb). Continuous variation of the junction resistance, spanning more than six orders of magnitude, results in the observation of the superconducting gap, Andreev reflection, and, for low resistance, a zero-bias sharp peak. Detailed study of the conductance curves and comparison with results for junctions composed of two superconducting electrodes support the interpretation of this feature as a second-order proximity-induced Josephson effect (PJE).

• Received 23 April 1992

DOI:https://doi.org/10.1103/PhysRevB.46.5814