Surface acoustic wave determination of the superconducting fraction of a Nb₃Ge film

Abstract

Surface acoustic waves at 1200 MHz were used to investigate the properties of a 0.5-µm-thick film of Nb₃Ge. This film was rf-sputtered onto a substrate consisting of a piezoelectrically active 3.5-µm AlN layer which was chemically vapor-deposited over a sapphire substrate. The attenuation coefficient α of the surface acoustic waves was measured from 0.8 to 30 K. The raw data are analyzed to eliminate interference effects due to splitting of the wave into two components. One of these is at the surface of the film and the other may be a “surface skimming bulk mode” which is at the film-substrate interface. The resultant curve of attenuation versus temperature in the superconducting region is then used to determine the distribution function of the superconducting transition temperature of the film. Although the film starts to become superconducting at 21 K and the majority of the film appears to become superconducting at 18 K, it is also found that a significant amount of the film does not become superconducting until 10 K. The difference between the attenuation measured in the normal state and the superconducting state is used to obtain the electron mean free path in the film. This is compared to values obtained from electrical measurements.