Analysis of CsV₃Sb₅ through microwave surface impedance

We employ resonant cavity perturbation techniques on CsV₃Sb₅ single crystals to measure the microwave surface impedance of this Kagome superconductor. This experiment enables us to characterize the microwave loss in this material through the surface resistance and the closely related real part of the complex conductivity σ₁, which, to our knowledge, are not reported in the literature. By studying the temperature dependence of the surface impedance and other electrodynamic quantities which follow from it, we are able to investigate the nature of the superconducting energy gap. Additionally, the sensitivity of this experiment to the loss enables possible insights into residual loss mechanisms such as a charge density wave. We consider nodal (power-law) as well as fully gapped (exponential) models for the penetration depth at low temperatures (T<Tc/3). We additionally observe power-law behavior of the surface resistance in this temperature range. We compare σ₁ to the Mattis-Bardeen prediction for one and two gaps over the full superconducting state to determine if a coherence peak is present in our data. We find power-law exponents α>2 for the penetration depth and surface resistance across many modes. Results for a single gap give Δ~0.1-0.5 meV, which is broadly consistent with values from the literature. Analyses with two gaps result in values for the lower gap Δ₁ which fall in a similar range to those of the single gap results while values for the upper gap are possibly, unreasonably large Δ₂>2 meV (6.4 k_BT_c).