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.