A possible technique for determining the average grain size in thin granular superconducting films

A new technique is proposed by which it may be possible to determine the average grain size of a thin superconducting film from measurements of the electrical resistance and the surface acoustic wave (SAW) attenuation. This technique is based on a percolation model for the SAW attenuation and resistance in a granular superconducting film. In essence, the model claims that as the grain size becomes comparable to the SAW wavelength, one should observe differences between the measured SAW attenuation and the attenuation predicted by acoustoelectric coupling of the SAW to the film. By measuring the SAW attenuation and comparing it the theoretically predicted value for the acoustoelectric attenuation (which is proportional to the normal state resistance of the film), one can determine the average grain size of the film. This technique would have two main advantages over conventional techniques used to measure grain size. First, the technique would not alter or destroy the sample. Secondly, the technique could be adapted to make in-situ measurements of the grain size of the film. In this paper we will review the underlying theory and present experimental data which gives qualitative support for the use of this method.