Abstract
A negative-resistance region has been observed just above the gap in tunnel junctions fabricated on 110>-oriented single-crystal Pb films. This phenomenon was found to be present only when tunneling into single-crystal Pb films about 1.4 \ensuremath{\mu}m thick. When these junctions were raised to temperatures above 3.7\ifmmode^\circ\else\textdegree\fi{}K or placed in a parallel applied magnetic field above 200 G the negative resistance was not present although a deep minimum in conductance persisted. The structure was always accompanied by large Tomasch oscillations and appears to be related to the presence of the Tomasch effect and the details of the effect as they occur in 100>-oriented single-crystal Pb films. The negative resistance and accompanying structure were reproduced in some respects by theoretical calculations using a superconductor density of states modified by the Tomasch term.
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