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

Abstract

The optimum design of high-sensitivity Superconducting Quantum Interference Devices (SQUIDs) and other devices based on thin 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 is largely missing in the literature. We report here inductance measurements on a set of 20 different thin-film YBa 2 Cu 3 O 7−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 pick-up loops. The latter agree to better than 7% with experimentally-measured values, validating our measured value of λ (77) and our inductance extraction methods.

Modelling high- Tc electronics

“…There has been a number of measurements of λ at a fixed temperature [56,57] by directly measuring the inductance of part of a SQUID loop using current injection. Very recent measurements [58] to find λ (77) and λ(T) for 50 < T < 79 K have been made on SQUIDs like figure 10: These are the type used in directly-coupled magnetometers [59] but here no magnetometer loop is present. A current I is injected through the blue part of the loop and from the flux change measured by the SQUID itself its partial inductance L c is found.…”

“…But it is also desirable to know λ(T) over the range attainable by small cryocoolers, as these are being increasingly used to cool devices below 77 K to enhance performance. The lack of published data for λ(T) prompted Keenan et al [58] to make cryocooler measurements of L co (T) on three different SQUIDs L co was then extracted by 3D-MLSI, stepping λ from 200 to 500 nm, figure 11(b). This data was merged with the experimental values of L co (T) to create a dataset for λ(T).…”

“…Co-planar striplines as shown in figure B1 are commonly used to form so-called linear SQUIDs (e.g. [57,58,122]) and also as low-inductance interconnects between parts of single-layer devices.…”

The effect of bias current configuration on the performance of SQUID arrays

“…Here we study three scenarios: the cell inductance is purely geometric (κ = 0), the kinetic and geometric inductances are equal (κ = 0.5), and the kinetic inductance dominates (κ = 0.9). The case κ = 0 corresponds to a LTS device, while κ = 0.5 corresponds to a HTS device [10,12]. with black circles), κ = 0.5 (lines with purple diamonds) and κ = 0.9 (lines with orange squares).…”

“…Arrays of superconducting quantum interference devices (SQUIDs) have been studied as a means of improving the magnetic field sensitivity [1], noise performance [2], dynamic range [3,4], linearity [5] and robustness [6] over that of a single dc-SQUID [7,8]. The performance of SQUID arrays depends on a range of parameters, including the device geometry [9,10] and superconducting film properties [11,12], * Authors to whom any correspondence should be addressed.…”