2-LEVEL TUNNEL RESISTANCE FLUCTUATIONS AT MECHANICALLY CONTROLLABLE BREAK JUNCTIONS IN HE EXCHANGE GAS

The tunnel resistance between two metallic electrodes of a mechanicall y controllable break junction, measured as a function of electrode sep aration, displays a clear deviation from exponential behavior when He is adsorbed at the electrodes at low temperatures. At T = 1.2 K, two-l evel resistance fluctuations occur in a limited resistance range which are attributed to transfers of He atoms between adsorption potential minima. The transfer rate varies with the applied bias voltage to a po wer p, which is between 1.3 and 1.7 for He-4, and about 2 for He-3. Th is bias dependence may be due to inelastic interactions between tunnel ing electrons and the adsorbed He. (C) 1998 Elsevier Science B.V. All rights reserved.