Exploring the performance of thin-film superconducting multilayers as Kinetic Inductance Detectors for low-frequency detection

TL;DR

This paper numerically analyzes Ti-Al thin-film multilayers to optimize Kinetic Inductance Detectors for low-frequency photon detection, demonstrating potential for high quality factors and long quasiparticle lifetimes.

Contribution

It introduces a numerical method to evaluate the superconducting properties of Ti-Al multilayers for KIDs, guiding design for low-frequency detection.

Findings

Differences in pair-breaking thresholds between bilayers and trilayers.

Predicted high quality factors for multilayer KIDs.

Long quasiparticle recombination times compared to pure Al films.

Abstract

We have solved numerically the diffusive Usadel equations that describe the spatially-varying superconducting proximity effect in Ti-Al thin-film bi- and trilayers with thickness values that are suitable for Kinetic Inductance Detectors (KIDs) to operate as photon detectors with detection thresholds in the frequency range of 50-90 GHz. Using Nam's extension of the Mattis-Bardeen calculation of the superconductor complex conductivity, we show how to calculate the surface impedance for the spatially varying case, and hence the surface impedance quality factor. In addition, we calculate energy-and spatially-averaged quasiparticle lifetimes at temperatures well-below the transition temperature and compare to calculation in Al. Our results for the pair-breaking threshold demonstrate differences between bilayers and trilayers with the same total film thicknesses. We also predict high quality factors and long multilayer-averaged quasiparticle recombination times compared to thin-film Al. Our calculations give a route for designing KIDs to operate in this scientifically-important frequency regime.