ELECTRODE GEOMETRY INFLUENCING THE TUNNEL RESISTANCE IN A MECHANICALLY CONTROLLABLE BREAK JUNCTION

In the conventional tunneling picture, the dependence of the tunnel re sistance on the distance between two electrodes in vacuum is expected to be exponential. We have observed, however, clear deviations from th is behavior over four to six orders of magnitude of the tunnel resista nce at constant biases of 5-100 mV, using highly stable mechanically c ontrollable break junctions of Al, Ag, Au, Pt, and PtIr. Previous theo retical studies indicate that deviations from exponential behavior can be caused by interactions between the two electrodes, as well as by c ertain electrode geometries. Since the first are predicted to influenc e the tunnel resistance over at most two orders of magnitude in the re gion close to contact, whereas the latter may extend over more than fo ur orders of magnitude, we believe to have observed effects due to sev eral different types of geometries.