The Surface Resistance of Superconducting A15 Niobium-Tin Films at 8.6 GHZ
A calorimetric technique for measuring the temperature dependence of the surface resistance of high-T(,c) superconducting thin films at 8.6 GHz has been developed. The technique has been applied to study electron-beam, co-deposited films of the A15 phase of Niobium-Tin with the goal of optimizing the microwave losses of the material. In addition, films of sputtered Niobium-Tin, a Niobium-Zirconium alloy, and Niobium-Nitride were also studied. For the evaporated Niobium-Tin films, carefully controlled deposition temperatures of greater than 900 C are necessary to obtain the lowest losses. A sharp transition is observed for stoichiometric material (25 percent Tin), but for the understoichiometric compositions the transitions are wider, yielding excessive losses in the material. Films prepared by magnetron sputtering behave similarly. A procedure, phase-locking, for preparing the stoichiometric composition which does not require exact control of the deposition rates has been developed and successfully demonstrated with the evaporated films. The experimental data are compared with two theoretical predictions: one for the superconducting state based on the BCS theory and a normal-state calculation in the classical skin-depth limit. When the residual losses (10 micro-ohms) are subtracted from the data, the behavior predicted for the superconducting state is observed. The normal-state losses, however, are anomalously large for the Niobium -Tin films. Possible explanations of this result are discussed. Reduced gaps are also obtained for the samples from their low temperature surface resistance. Values obtained for the Niobium-Tin films are lower than those from other measurements and may be due to poor material near the substrate interface. Even at the present level of material development, the prognosis for the application of Niobium-Tin films to microwave devices appears favorable. When compared with Niobium, the most common material choice, Niobium -Tin films as prepared by phase-locking achieve the same performance at 10 K as Niobium does at 5 K. With the operating temperatures of miniature refrigerators approaching 10 K, a technology based on Niobium-Tin that avoids liquid helium would thus be feasible.
- Pub Date:
- MICROWAVE LOSSES;
- Physics: Condensed Matter