Penetration depth, microwave surface resistance, and gap ratio in NbN and Ba1−xKxBiO3 thin films

We report values of the zero temperature magnetic penetration depth λ(0), microwave surface resistance Rs, and gap ratio 2Δ(0)/kBTc in technologically useful thin films of NbN and Ba1−xKxBiO3. A novel analysis technique was used to extract the absolute magnitude of λ(0) and 2Δ(0)/kBTc from shifts in resonant frequency of a parallel-plate resonator. For NbN and Ba1−xKxBiO3 values of λ(0)=3900±200 Å and 3300±200 Å were obtained, respectively. The gap ratios were found to be 2Δ(0)/kBTc=4.1±0.1 and 3.8±0.5, respectively, for Tc=16.3 K in NbN and Tc=17.2 K in Ba1−xKxBiO3. The surface resistance measurements on Ba1−xKxBiO3 represent the lowest values ever reported at microwave frequencies in this material.