CONDUCTANCE AND SUPERCURRENT DISCONTINUITIES IN ATOMIC SIZE POINT CONTACTS

A technique is described, which uses the adjustable stress in a bending beam to control the constriction diameter between two electrodes. The controllability of the constriction diameter is useful when a large range in the normal resistance of different junctions is required for a systematic study. In the high resistance point contact regime quantum size effects are studied in metallic and superconducting constrictions. When changing the constriction diameter reproducible steps in the conductance of the order of 2e2/h are observed. For superconducting point contacts concomitant jumps in the supercurrent of order eDELTA/HBAR are seen. For contacts adjusted at a step the conductance switches in time between two values, which are interpreted as ''two level fluctuations'' in the site of a single atom in the constriction.