Field response of ultra-thin type II superconducting transmission lines

This work describes the effect of a tangential magnetic field on the superconducting penetration depth, /spl lambda/, as modeled by the theory of Ginzburg and Landau. In particular, an increasing magnetic field decreases the magnitude of the order parameter /spl Psi/. Consequently, the London equations have been modified to include field dependent values of the penetration depth, /spl lambda/(T,H) and complex conductivity, /spl sigma/(T,H). The analysis assumes that the superconducting films are free of vortices. For Type II films, this criteria can only be met when the thickness of the conductors is less than 1.8/spl xi//sub GL/. The G-L theory is used to find closed form solutions that describe the electromagnetic characteristics of a kinetic inductance transmission line including phase velocity, impedance, and energy loss, as functions of power, temperature and magnetic fields. Limitations of power are also discussed. To validate these concepts, experiments verifying the effects of temperature and bias field were implemented using niobium superconductors 500 /spl Aring/ thick separated by 1000 /spl Aring/ of Al/sub 2/O/sub 3/. Potential device applications include variable phase-shifters, tunable filters, and extremely sensitive bolometers.<>