Marc Scheffler

Characterization of harmonic modes and parasitic resonances in multi-mode superconducting coplanar resonators

  1. Cenk Beydeda,
  2. Konstantin Nikolaou,
  3. Marius Tochtermann,
  4. Nikolaj G. Ebensperger,
  5. Gabriele Untereiner,
  6. Ahmed Farag,
  7. Philipp Karl,
  8. Monika Ubl,
  9. Harald Giessen,
  10. Martin Dressel,
  11. and Marc Scheffler
Planar superconducting microwave transmission line resonators can be operated at multiple harmonic resonance frequencies. This allows covering wide spectral regimes with high sensitivity,
as it is desired e.g. for cryogenic microwave spectroscopy. A common complication of such experiments is the presence of undesired ’spurious‘ additional resonances, which are due to standing waves within the resonator substrate or housing box. Identifying the nature of individual resonances (‚designed‘ vs. ’spurious‘) can become challenging for higher frequencies or if elements with unknown material properties are included, as is common for microwave spectroscopy. Here we discuss various experimental strategies to distinguish designed and spurious modes in coplanar superconducting resonators that are operated in a broad frequency range up to 20 GHz. These strategies include tracking resonance evolution as a function of temperature, magnetic field, and microwave power. We also demonstrate that local modification of the resonator, by applying minute amounts of dielectric or ESR-active materials, lead to characteristic signatures in the various resonance modes, depending on the local strength of the electric or magnetic microwave fields.
arxiv pdf

Characterizing dielectric properties of ultra-thin films using superconducting coplanar microwave resonators

  1. Nikolaj G. Ebensperger,
  2. Benedikt Ferdinand,
  3. Dieter Koelle,
  4. Reinhold Kleiner,
  5. Martin Dressel,
  6. and Marc Scheffler
We present an experimental approach for cryogenic dielectric measurements on ultra-thin insulating films. Based on a coplanar microwave waveguide design we implement superconducting
quarter-wave resonators with inductive coupling, which allows us to determine the real part ε1 of the dielectric function at GHz frequencies and for sample thicknesses down to a few nm. We perform simulations to optimize resonator coupling and sensitivity, and we demonstrate the possibility to quantify ε1 with a conformal mapping technique in a wide sample-thickness and ε1-regime. Experimentally we determine ε1 for various thin-film samples (photoresist, MgF2, and SiO2) in the thickness regime of nm up to μm. We find good correspondence with nominative values and we identify the precision of the film thickness as our predominant error source. Additionally we demonstrate a measurement of ε1(T) vs. temperature for a SrTiO3 bulk sample, using an in-situ reference method to compensate for the temperature dependence of the superconducting resonator properties.
arxiv pdf