Superfluid density in 2D organic superconductors: evidence for d-wave pairing

Organic superconductors are an exciting ``playground'' for low dimensional physics, with a clean, layered structure that exhibits a variety of electronic phases including superconductivity. The interactions responsible for pairing, and the symmetry of the pair wavefunction, continue to be open issues in these materials. To gain further insights, we have carried out microwave spectroscopy of two BEDT-TTF-based superconductors. Penetration depth measurements reveal a strong, linear temperature dependence of superfluid density, indicating line nodes in the order parameter and providing strong evidence for $d$-wave pairing. Measurements of the microwave conductivity allow us to extract the quasiparticle scattering rate, both above \textit{and} below $T_{c}$. In the normal state, the scattering is strong, at several times the thermal energy. Below $T_{c}$ there is a rapid drop in scattering, with a $T^{3}$ temperature dependence characteristic of $d$-wave quasiparticles scattering from antiferromagnetic spin fluctuations.