Electrical conduction of thin metallic layers at high frequencies down to liquid helium temperatures

Abstract

The dependence of electrical conduction on current frequency has been studied in thin layers of pure metals at temperatures between 4.2°K and 297°K. The experimental data are given for frequencies up to 2 × 10 6 Hz. The thin layers were made by deposition from vapour on to cover glass substrates in a vacuum of 10 −6mm of Hg. As shown by the experimental data, the decrease in electrical resistance with the increase of the frequency is more pronounced in layers having large dc resistance. In layers with negative temperature coefficients of resistance, this decrease is more rapid at low temperatures. The relation between the relative change in electrical conductance and the frequency drawn on a double logarithmic scale is a straight line with a slope equal to 2. On the assumption of a granular structure of the layers, an interpretation is suggested for these properties using an equivalent electric circuit consisting of an intergranular resistance, a bulk resistance of the metallic grains, and an intergranular capacitance which depends on the gaps between the grains. In some very thin layers the ac resistance diminishes continuously with the increasing frequency tending towards the value zero. The origin of this anomalous effect can be ascribed to stray capacitances. The estimation of the ratio between bulk and intergranular resistances discussed from the viewpoint of this theoretical circuit is compared with data from magnetoresistance measurements on ferromagnetic layers.