Coupling at RF frequencies in the superconducting film transformer

Two superconducting oxygen-doped aluminum films, separated by a silicon monoxide insulating film, contain two magnetically coupled vortex lattices in a transverse applied magnetic field. The rf impedance of the device shows structure that is sensitive to details of the magnetic coupling force, which is a periodic function of the relative displacement of the two vortex lattices. When the lattices are driven at nearly equal dc velocities, the observed rf response is a function of the coupling force, itself externally controlled by the dc currents applied to the films. Rf measurements were used to obtain the relaxation time of the coupling mechanism, which is inversely proportional to the gradient of the coupling force. By integration, one obtains the displacement dependence of the coupling force. Results of a recent theory by Clem are in good agreement with the data.