Thermally assisted flux flow excited by ultrasound in superconducting thin films

A new ultrasonic method has been proposed for the investigation of the depinning line of superconducting thin films. The motion of the depinned flux lines is detected as the excess flux-flow voltage caused by the excitation of pinned flux lines through the ultrasonic vibration of pinning centers. When a strain related to the tilt modulus C44 of flux-line lattice was applied to YBa2Cu3O7-δ thin films in a parallel magnetic field, a maximum of the excess flux-flow voltage was observed below the superconducting transition temperature. To explain the maximum observed, a theory is proposed in which thermally assisted flux flow and ultrasonic attenuation due to the depinning of flux lines are taken into account. The discrepancies between the experimental and the theoretical results are discussed with the ultrasonic frequency and amplitude dependence of the depinning temperature.