Possible Ultrasonic Evidence for the Existence of Vortex-Antivortex Pairs

The attenuation of surface acoustic waves in the 700 MHz frequency range passing through 1000 Å NbN films has been measured on several NbN films. The films have a columnar structure where the columns are about 200 Å in diameter separated by 20 Å voids. In the superconducting state the attenuation does not follow the usual BCS curve. It appears to be composed of the sum of a BCS curve plus another curve which has a maximum below the superconducting transition temperature. The attenuation data may also be analyzed to yield an effective energy gap which is quenched at about one fifth of the BCS zero temperature energy gap. Tentatively, the Kosterlitz-Thouless vortex-antivortex model is used to determine the temperature dependence of an effective order parameter that yields reduced attenuation data which agree qualitatively with the experimental results.