III. Alkali films with the properties of the normal metal

Abstract

In the present work the measurements of the resistivity of evaporated films of the alkali metals (Lovell 1936 a, b ; Appleyard and Lovell 1937) have been extended to a thickness of several thousand angstroms. The previous results were limited to films of less than a few hundred angstroms in thick­ness; and it appeared that thin creased resistivity of these thin films com­pared with that of the bulk metal was due merely to the shortening of the mean free path of the conduction electrons by collision with the boundaries of the film. It was therefore to be expected that at thicknesses much in excess of the mean free path the resistivity of the film material should closely approximate to that of the bulk metal. The present results confirm this expectation only in the case of caesium, where the resistivity approaches within a few per cent that of the bulk metal. Potassium and rubidium films, on the other hand, show a considerable excess of resistivity above that of the bulk metal, but it is shown that this excess is a residual resistance due to lattice strain, and that it may be partially removed by suitable annealing.