Single Superconducting Split-Ring Resonator Electrodynamics
  We investigate the microwave electrodynamic properties of a single superconducting thin film split-ring resonator (SRR). The experiments were performed in an all-Nb waveguide, with Nb wires and Nb SRRs. Transmission data showed a high-Q stopband for a single Nb SRR ($Q \sim 4.5\times10^4$ at 4.2 K) below $T_c$, and no such feature for a Cu SRR, or closed Nb loops, of similar dimensions. Adding SRRs increased the bandwidth, but decreased the insertion loss of the features. Placing the Nb SRR into an array of wires produced a single, elementary negative-index passband ($Q \sim 2.26\times10^4$ at 4.2 K). Changes in the features due to the superconducting kinetic inductance were observed. Models for the SRR permeability, and the wire dielectric response, were used to fit the data. 
Michael C. Ricci and Steven M. Anlage