Novel technique to measure the microwave response of high T c superconductors between 4.2 and 200 K

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Abstract
We have devised and implemented a novel technique that has the required sensitivity to enable measurements of the complex surface impedance (Zs) of high Tc superconducting materials between 4.2 K and at least 200 K. The essential ideal is to employ a superconducting high-Q cavity resonator operated at an ambient temperature of 4.2 K. The sample, mounted on a sapphire rod, is placed inside the cavity at a high magnetic field location, and is thermally insulated from the cavity walls, enabling external control of the sample temperature between 4.2 and 200 K. The cavity characteristics are dominated by the sample properties—the Pb walls maintained at 4.2 K contribute negligibly. Measurement of the cavity Q and resonant frequency enables the measurement of Zs as a function of the sample temperature. The technique is applicable to both bulk and thin-film materials. We have used this technique with success to measure Zs, at 9.58 GHz, for bulk Y1Ba2Cu3Oy and La1.85Sr0.15CuO4 over a temperature range from 4.2 to 100 K.