High-precision measurement of magnetic penetration depth in superconducting films

The magnetic penetration depth ( λ) in thin superconductingfilms is usually measured by the mutualinductance technique. The accuracy of this method has been limited by uncertainties in the geometry of the solenoids and in thefilm position and thickness, by parasitic coupling between the coils, etc. Here, we present several improvements in the apparatus and the method. To ensure the precise thickness of the superconducting layer, we engineer thefilms at atomic level using atomic-layer-by-layer molecular beam epitaxy. In this way, we also eliminate secondary-phase precipitates, grain boundaries, and pinholes that are common with other deposition methods and that artificially increase the field transmission and thus the apparentλ. For better reproducibility, the thermal stability of our closed-cycle cryocooler used to control the temperature of the mutualinductance measurement has been significantly improved by inserting a custom-built thermal conductivity damper. Next, to minimize the uncertainties in the geometry, we fused a pair of small yet precisely wound coils into a singlesapphire block machined to a high precision. The sample is spring-loaded to exactly the same position with respect to the solenoids. Altogether, we can measure the absolute value ofλ with the accuracy better than ±1%.
Source: Review of Scientific Instruments - Category: Physics Authors: Source Type: research