Determining the temperature-dependent London penetration depth in HTS thin films and its effect on SQUID performance

Keenan, Shane and Pegrum, Colin and Gali Labarias, Marc and Mitchell, Emma E. (2021) Determining the temperature-dependent London penetration depth in HTS thin films and its effect on SQUID performance. Applied Physics Letters, 119 (14). 142601. ISSN 0003-6951 (

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The optimum design of high-sensitivity Superconducting Quantum Interference Devices (SQUIDs) and other devices based on thin high-temperature superconductor (HTS) films requires accurate inductance modeling. This needs the London penetration depth λ to be well defined, not only at 77K, but also for any operating temperature, given the increasingly widespread use of miniature low-noise single-stage cryocoolers. Temperature significantly affects all inductances in any active superconducting device, and cooling below 77 K can greatly improve device performance; however, accurate data for the temperature dependence of inductance and λT for HTS devices are largely missing in the literature. We report here inductance measurements on a set of 20 different thin-film YBa 2Cu 3O 7−x SQUIDs at 77K with thickness t=220 or 113nm. By combining experimental data and inductance modeling, we find an average penetration depth λ 77=nm at 77K, which was independent of t. Using the same methods, we derive an empirical expression for λ for a further three SQUIDs measured on a cryocooler from 50 to 79=K. Our measured value of λ7 and our inductance extraction procedures were then used to estimate the inductances and the effective areas of directly coupled SQUID magnetometers with large washer-style pickup loops. The latter agrees better than 7% with experimentally measured values, validating our measured value of λ and our inductance extraction methods.