Tunnelspektroskopie an dünnsten supraleitenden Aluminium-, Indium- und Bleischichten / Superconducting Tunneling into Ultrathin Aluminum, Indium and Lead Films

Abstract

We report about tunneling experiments on extremely thin superconductors with barriers of aluminium- oxide and in addition silicon-monoxide or naphthalene, all condensed at 4.2 °K. The tunneling spectra of aluminium, indium and lead films show a large tunneling anomaly at a film thickness just above the critical thickness for conductivity and superconductivity: in the low bias range (between 2 and 5 mV) the tunneling resistivity dU/dI is increased by about 20 to 30 per cent with respect to the corresponding resistance of thicker films. This anomaly disappears when the film thickness is increased by 30 to 40 Å. Two models are presented, which can explain the tunneling anomaly by the quantum size effect in the very thinnest films of either the phonon states or the electronic states. The energy gaps and the transition temperatures of the superconductors show a marked dependence on the film thickness between the critical film thickness and 300 Å, dependent on both, the superconductor itself and on the substrate, naphthalene or silicon-monoxide. This dependence can be described completely by the altered electron-phonon coupling at the film surfaces and by size quantization of the electronic states.