Surface Impedance of Thin Superconducting Films
<p>Theoretical analysis and experimental measurements have been made of the propagation of electromagnetic waves in a structure consisting of two planar superconductors which are of the order of a penetration depth apart. One superconductor is tantalum and is much thicker than a penetration de...
| Summary: | <p>Theoretical analysis and experimental measurements have been made of the propagation of electromagnetic waves in a structure consisting of two planar superconductors which are of the order of a penetration depth apart. One superconductor is tantalum and is much thicker than a penetration depth; the other is a vacuum evaporated indium film and may be as thin as a penetration depth.</p>
<p>It is shown that such a structure will propagate waves at a phase velocity less than the speed of light in the medium separating the superconductors, a phenomenon that is the result of an inductive component in the surface impedance of the superconductors. The exact velocity is shown to be a function of the thickness parameters in a manner which depends on the law relating the vector potential and the supercurrent in the indium.</p>
<p>Experimental measurements indicate that the relationship between vector potential and current in the vacuum evaporated indium is characterized by a coherence distance which is considerably smaller than that found for pure metals by the measurements of Pippard and the theory of Bardeen, Cooper and Schrieffer.</p>
<p>The penetration depth at zero temperature is deduced from dependence of phase velocity on the thicknesses of the indium and dielectric. For indium λ is found to be 650 ± 75Å, in good agreement with Lock's value of 640Å and Toxen's range from 625 to 725Å. For tantalum λ is found to be 500 ± 175Å. This is believed to be the first measurement. The value of λ for indium is also deduced from the dependence of phase velocity on temperature. It is found to be 704 ± 120Å.</p>
<p>Surface resistance of the two superconductors is found to increase ω<sup>2</sup>, in good agreement with theory, and to depend on temperature according to an empirical law proposed by Pippard.</p>
|
|---|