Superconducting penetration depth of aluminum thin films

We present a study of the superconducting penetration depth λ in aluminum thin films of varying thickness. The range of thicknesses chosen spans from the thin-film regime to the regime approaching bulk behavior. The penetration depths observed range from λ = 163 ± 1 nm for the thinnest 28 nm samples down to λ = 58 ± 1 nm for the 207 nm-thick ones, allowing us to provide an estimate of the thickness at which aluminum becomes a type-I superconductor. In order to accurately determine λ , we performed complementary measurements using the frequency of superconducting LC resonators obtained through novel and efficient methods of fitting and simulation, as well as the normal-state resistance of meandered structures. Both methods yield comparable results, providing a well-characterized set of values of λ in aluminum in the relevant range for applications in fields such as quantum computing and microwave radiation detector technologies.