A current controlled variable delay superconducting transmission line

The authors present a device concept for a superconductive current-biased variable-delay transmission line structure which is capable, in principle, of operating up to the terahertz regime. The device makes use of the change in kinetic inductance of superconductors with transport current. The relevant material figures of merit for optimum performance of such a device are defined, and suitable candidate materials are identified. The device concept has been tested in niobium technology, where temperature-dependent changes in the inductance are easily achieved. Preliminary measurements on the temperature and current dependence of niobium transmission line resonators operating in the 1-20 GHz range are presented. The expected DC bias current variable delay has not yet been observed, but niobium is not expected to be the optimum material for such an effect. Suggested improvements include the use of more favorable materials, such as amorphous alloys and oxide superconducting films, and the use of modified microstrip geometries where a closer approach to the depairing critical current density should be possible