[en] Planar superconducting resonators have multiple applications, e.g. in quantum information processing or spectroscopy on unconventional solids. Here, Pb can be used as a basis of microwave resonators with a critical temperature of 7.2 K and a critical magnetic field of 80 mT. It shows interesting behavior with residual normal conducting areas after entering the superconducting phase with applied external magnetic field. We present a study of superconducting Pb microwave stripline resonators in variable external magnetic field up to 140 mT and a temperature range of 1.6 K to 6.5 K. We are able to determine the complex conductivity of Pb in variable magnetic field and we find striking similarities to measurements in variable temperature, like a coherence peak. The quality factor of the resonances shows strong hysteresis effects after exceeding the critical magnetic field, indicating residual normal conducting areas in the resonator persisting in the superconducting state. We are able to determine the critical magnetic field for a set of given temperatures both for the Pb resonator as well as a Sn sample placed on top of the underlying Pb resonator. These results demonstrate that Pb resonators can be applied for different spectroscopy studies in zero as well as finite magnetic field.

[en] The electronic properties of superconducting Sn films (T_c ∼ 3.7 K) change significantly when lowering the film thickness down to a few nm, in particular at the percolation threshold. The low energy electrodynamics of such Sn samples can be probed via microwave spectroscopy, e.g. with superconducting stripline resonators. We have deposited Sn films by thermal evaporation, ranging in thickness between 38 nm and 842 nm, and we characterized their morphology by AFM. We use superconducting Pb stripline resonators to probe the microwave response of Sn films at temperatures from 7.5 K down to 1.5 K in a frequency range between 1 GHz and 20 GHz. The measured quality factor of the resonators decreases with increasing temperature due to increasing losses. As a function of the sample thickness we observe three regimes with significantly different properties: Samples below percolation 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, on the other hand, lead to low-loss resonances above and below T_c of Sn, but in an intermediate thickness regime, just above percolation, the metallic state of the Sn films is too lossy for resonator operation whereas the superconducting state only has low microwave losses.

[en] Planar superconducting microwave resonators are key elements in a variety of technical applications and also act as sensitive probes for microwave spectroscopy of various materials of interest in present solid state research. Here superconducting Pb is a suitable material as a basis for microwave stripline resonators. To utilize Pb stripline resonators in a variable magnetic field (e.g. in ESR measurements), the electrodynamics of such resonators in a finite magnetic field has to be fully understood. Therefore we performed microwave transmission measurements (with ample applied power to work in linear response) on superconducting Pb stripline resonators in a variable, parallel magnetic field. We determined surface resistance, penetration depth, as well as real and imaginary parts, ${\mathrm{\sigma <!--\; ??\; -->}}_1$ and ${\mathrm{\sigma <!--\; ??\; -->}}_2$, of the complex conductivity of superconducting Pb as a function of a magnetic field. Here we find features reminiscent of those in temperature-dependent measurements, such as a maximum in ${\mathrm{\sigma <!--\; ??\; -->}}_1$ (coherence peak). At magnetic fields above the critical field of this type-I superconductor we still find a low-loss microwave response, which we assign to remaining superconductivity in the form of filaments within the Pb. Hysteresis effects are found in the quality factor of resonances once the swept magnetic field has exceeded the critical magnetic field. This is due to normal conducting areas that are pinned and can therefore persist in the superconducting phase. Besides zero-field-cooling we show an alternative way to eliminate these even at $T<<!--\; <\; -->T_c$. Based on our microwave data, we also determine the critical magnetic field and the critical temperature of Pb in a temperature range between 1.6 K and 6.5 K and magnetic fields up to 140 mT, showing good agreement with BCS predictions. We also study a Sn sample in a Pb resonator to demonstrate the applicability of superconducting Pb stripline resonators in the experimental study of other (super-)conducting materials in a variable magnetic field. (paper)

[en] The electronic properties of superconducting Sn films ($T_{\mathrm{c}}\approx <!--\; ???\; -->3.8\mathrm{K}$) change significantly when reducing the film thickness down to a few $\mathrm{n}\mathrm{m}$, 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 $842\mathrm{n}\mathrm{m}$—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 $20\mathrm{G}\mathrm{H}\mathrm{z}$ at temperatures from $7.2$ down to $1.5\mathrm{K}$. 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. (paper)