Abstract
The electronic properties of superconducting Sn films (T-c approximate
to 3.8 K) change significantly when reducing the film thickness down to
a few nm, in particular close to the percolation threshold. The
low-energy electrodynamics of such Sn samples can be probed via
microwave spectroscopy, e.g. with superconducting stripline resonators.
Here we study Sn thin films, deposited via thermal evaporation-ranging
in thickness between 38 and 842 nm-which encompasses the percolation
transition. We use superconducting Pb stripline resonators to probe the
microwave response of these Sn films in a frequency range between 4 and
20 GHz at temperatures from 7.2 down to 1.5 K. The measured quality
factor of the resonators decreases with rising temperature due to
enhanced losses. As a function of the sample thickness we observe three
regimes with significantly different properties: samples below
percolation, i.e. ensembles of disconnected superconducting islands,
exhibit dielectric properties with negligible losses, demonstrating that
macroscopic current paths are required for appreciable dynamical
conductivity of Sn at GHz frequencies. Thick Sn films, as the other
limit, lead to low-loss resonances both above and below T-c of Sn, as
expected for bulk conductors. But in an intermediate thickness regime,
just above percolation and with labyrinth-like morphology of the Sn, we
observe a quite different behavior: the superconducting state has a
microwave response similar to the thicker, completely covering films
with low microwave losses; but the metallic state of these Sn films is
so lossy that resonator operation is suppressed completely.
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