In-plane superfluid density and microwave conductivity of the organic superconductor \kappa-(BEDT-TTF)2Cu[N(CN)2]Br: evidence for d-wave pairing
We report measurements of the in-plane microwave surface impedance of a high quality single crystal of the title compound. This yields three independent lines of evidence for d-wave pairing: i) a strong, linear temperature dependence of superfluid density; ii) deep in the superconducting state the quasiparticle scattering rate \Gamma(T) ~ T^3; and iii) no BCS conductivity coherence peak is observed in the quasiparticle conductivity. The zero-temperature penetration depth, \lambda_0 = 3220 \AA, is found to be much shorter than in previous reports. We also measure the in-plane conductivity above the superconducting critical temperature and use this to test theories of the normal state.