Determination of the kinetic inductance fraction in granular-aluminum microwave resonators

Abstract

We have designed, fabricated and characterized microwave resonators made out of granular aluminium (grAl) thin films with different oxygen content. The kinetic inductance fraction of these disordered superconductors was determined using electromagnetic finite-element simulations, conformal mapping techniques, and the frequency shift of the resonators with temperature. We compare these methods to assess model dependent variations and to highlight their main limitations. This analysis aims to guide the community in choosing the most suitable method for designing Kinetic Inductance Detectors (KIDs), high impedance resonators and superinductors. Additionally, we measured the internal quality factor as a function of microwave power at low temperature to characterize losses, which we identify as arising from two-level systems.