Microwave spectroscopy of organic superconductors

Organic superconductors, discovered in 1979, continue to be of immense interest in condensed matter physics because they provide a clean realization of low dimensional electronic systems in which kinetic and potential energies are finely balanced. Of particular interest in our context is the need to reconcile contradictory evidence regarding the symmetry of the Cooper pair wave function. High resolution microwave spectroscopy has been used to carry out electrodynamic measurements on single crystals of κ-(BEDT-TTF2Cu[N(CN)2]Br and κ-(BEDT-TTF)2Cu(SCN)2. Cavity perturbation measurements were carried out at a frequency of 2.91 GHz at temperatures down to 0.1 K. A microwave magnetic field was applied perpendicular to the conducting planes to induce in-plane screening currents. In both materials, measurements of superfluid density reveal clear regimes of linear temperature dependence at intermediate temperatures with crossovers to higher-order power law dependence at low temperature. This result is consistent with d-wave superconductivity in the presence of strong disorder.