Electron-Tunneling Measurements on Lanthanum and Lanthanum-Lutetium Alloy Films

Abstract

Results of electron-tunneling measurements on evaporated films of fcc lanthanum and dhcp lanthanum-lutetium alloys are presented. The ratio of the zero-temperature energy gap to the temperature at which the energy gap vanishes for both the pure lanthanum and lanthanum-lutetium alloy samples varied from 3.41 to 3.58. If one fits the lowest-temperature data for the energy gap with a curve of the BCS temperature dependence, the values at intermediate temperatures fall below the weak-coupling BCS prediction. The conductance maxima for the (pure La)-${\mathrm{Al}}_2$ ${\mathrm{O}}_3$-Al diodes are larger than predicted by the weak-coupling BCS theory. The conductance maxima for the lutetium alloy samples are more nearly equal to the weak-coupling BCS values than are those of the pure samples. They were not significantly altered by the presence of small zero-voltage anomalies. Hence zero-voltage anomalies are not enhanced at temperatures below the superconducting transition temperature. No change in conductance as large as 0.1% was observed which could be associated with the second energy gap predicted by the multiband-superconductor theory of Kuper, Jensen, and Hamilton. Kondo's multiband-superconductor theory is consistent with the experimental results. It is shown that if the $f$ band in Kondo's theory is approximately 20 meV or more higher than the Fermi level, then Kondo's theory reduces to a single-parameter theory having a gap equation identical in form to the BCS gap equation.