Ultrasonic attenuation of 700-MHz surface acoustic waves in thin films of superconducting NbN

The temperature dependence of the ultrasonic attenuation of 700-MHz surface acoustic waves has been measured for nine films of superconducting NbN varying in thickness from 0.02 to 0.3 {μ}m. The films, sputter deposited on LiNbO₃ substrates, exhibited attenuation changes in the superconducting state which were very similar in each case. These changes did not, however, follow the predictions of Bardeen, Cooper, and Schrieffer. Instead, they began with a sharp decrease for 1 or 2 dB at the transition temperature followed by a leveling off which remained nearly constant until T0.5Tc. It then decreased linearly with temperature until again leveling off below 1.5 K. A tentative explanation is proposed in terms of the Kosterlitz-Thouless vortex-antivortex theory.