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

We report temperature-dependent thermal-conductivity, κ, measurements on the layered quasi-two-dimensional organic superconductors κ-(BEDT-TTF)Cu(NCS) and κ-(BEDT-TTF)Cu[N(CN)]Br down to 160 mK. The results for κ-(BEDT-TTF)Cu(NCS) may be consistent with a nodal superconducting (SC) gap structure as indicated by a non-negligible remnant linear contribution when [Formula: see text] is extrapolated to [Formula: see text]. For κ-(BEDT-TTF)Cu[N(CN)]Br, contrary to expectations, higher κ values are observed in the superconducting regime as compared to the normal, high-field state evidencing a dominant phonon contribution to κ in the superconducting state. The strong increase of κ in the normal state below T 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.