Ultrasonic surface acoustic wave investigation of thin films of superconducting NbN

Surface acoustic waves in the 700 MHz range were launched on two LiNbO3substrates across thin films of sputter deposited NbN 0.1 microns thick using photodeposited interdigital electrodes. The attenuation monitored as a function of temperature in the 1.2 to 20 K range exhibited a very rapid change of 1 to 2 dB at Tc. However as the temperature was lowered further the attenuation leveled off and began a much more gradual decrease which lasted for the rest of the range. The four-wire resistance of the film monitored simultaneously with a 10 μA current dropped to zero within 1 K of Tc. The data are analyzed in four different ways in order to extract information about a possible mechanism that is producing these unexpected results.