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

Shane Keenan,Colin Pegrum,Emma E Mitchell,Marc Gali Labarias

doi:10.1063/5.0065790

Abstract

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 77 K, 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 λ(T) for HTS devices are largely missing in the literature. We report here inductance measurements on a set of 20 different thin-film YBa2Cu3O7−x SQUIDs at 77 K with thickness t = 220 or 113 nm. By combining experimental data and inductance modeling, we find an average penetration depth λ(77)=391 nm at 77 K, which was independent of t. Using the same methods, we derive an empirical expression for λ(T) for a further three SQUIDs measured on a cryocooler from 50 to 79 K. Our measured value of λ(77) 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(77) and our inductance extraction methods.

Highlights

For thin-film Superconducting Quantum Interference Devices (SQUIDs) and related devices the inductances of all their parts need to be known at the design stage in order to optimize performance at a given temperature T
Li et al.21 measured Lsq for five nano-slit SQUIDs fabricated by a focused helium ion beam on a 25 nm thick YBCO film and found λ = 180 nm gave the best match to experiment at 9 K
We combined our experimental measurements of L0 with inductance extraction data from SQUID models in which λ is a variable parameter, to find λ (77) for the five different samples

Summary

Introduction

For thin-film SQUIDs and related devices the inductances of all their parts need to be known at the design stage in order to optimize performance at a given temperature T. This motivated us to make direct inductance measurements on a set of YBCO SQUIDs with different line-widths, loop sizes and film thicknesses and to combine these with inductance extraction techniques to find the penetration depth λ (77) at 77 K and an expression for λ (T ) in the range 50 < T < 78 K.

Results

Conclusion