Abstract
We report the in-plane microwave surface impedance of a high-quality single crystal of $\ensuremath{\kappa}$-(BEDT-TTF)${}_{2}$Cu[N(CN)${}_{2}$]Br. In the superconducting state, we find three independent signatures of $d$-wave pairing: (i) a strong, linear temperature dependence of superfluid density; (ii) deep in the superconducting state the quasiparticle scattering rate $\ensuremath{\Gamma}\ensuremath{\sim}{T}^{3}$; and (iii) no BCS coherence peak is observed in the quasiparticle conductivity. Above ${T}_{c}$, the Kadowaki-Woods ratio and the temperature dependence of the in-plane conductivity show that the normal state is a Fermi liquid below $\ensuremath{\simeq}23$ K, yet resilient quasiparticles dominate the transport up to $\ensuremath{\simeq}50$ K.
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