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The electronic properties of superconducting Sn films ($T_c \approx$ 3.8 K)change significantly when reducing the film thickness down to a few nm, inparticular close to the percolation threshold. The low-energy electrodynamicsof such Sn samples can be probed via microwave spectroscopy, e.g. withsuperconducting stripline resonators.  Here we study Sn thin films, deposited via thermal evaporation -ranging inthickness between 38 nm and 842 nm- which encompasses the percolationtransition. We use superconducting Pb stripline resonators to probe themicrowave response of these Sn films in a frequency range between 4 GHz and 20GHz at temperatures from 7.2 K down to 1.5 K. The measured quality factor ofthe resonators decreases with rising temperature due to enhanced losses. As afunction of the sample thickness we observe three regimes with significantlydifferent properties: samples below percolation, i.e. ensembles of disconnectedsuperconducting islands, exhibit dielectric properties with negligible losses,demonstrating that macroscopic current paths are required for appreciabledynamical conductivity of Sn at GHz frequencies. Thick Sn films, as the otherlimit, lead to low-loss resonances both above and below $T_c$ of Sn, asexpected for bulk conductors. But in an intermediate thickness regime, justabove percolation and with labyrinth-like morphology of the Sn, we observe aquite different behavior: the superconducting state has a microwave responsesimilar to the thicker, completely covering films with low microwave losses;but the metallic state of these Sn films is so lossy that resonator operationis suppressed completely.

Microwave study of superconducting Sn films above and below percolation