Investigation of the superconducting gap structure in κ-(BEDT-TTF)2Cu(NCS)2 and κ-(BEDT-TTF)2Cu[N(CN)2] Br by means of thermal-conductivity measurements

We report temperature-dependent thermal-conductivity,., measurements on the layered quasi-two-dimensional organic superconductors kappa-(BEDT-TTF)(2)Cu(NCS)(2) and kappa-(BEDT-TTF)(2)Cu[N(CN)(2)] Br down to 160 mK. The results for kappa-(BEDT-TTF)(2)Cu(NCS)(2) may be consistent with a nodal superconducting (SC) gap structure as indicated by a non-negligible remnant linear contribution when kappa/T alpha T-2 is extrapolated to T = 0. For kappa-(BEDT-TTF)(2)Cu[N(CN)(2)] Br, contrary to expectations, higher. values are observed in the superconducting regime as compared to the normal, high-field state evidencing a dominant phonon contribution to. in the superconducting state. The strong increase of. in the normal state below T-c for both samples indicates strong electron-phonon scattering. Our results highlight the need for thermal-conductivity measurements performed down to significantly lower temperatures to determine the symmetry of the SC gap.