Search for the superconducting gap structure in κ-(BEDT-TTF)2X by use of thermal-conductivity measurements

κ-ET₂X, where ET is bis(ethylenedithio)-tetrathiafulvalene (BEDT-TTF), is a family of organic charge transfer salts consisting of conducting ET layers interspersed between insulating X^- anions. For almost two decades it has been debated whether these quasi-two-dimensional (Q2D) superconductors are conventional BCS-type or unconventional d-wave similar to the cuprates. A variety of measurements techniques, including NMR, specific heat, and STM, have been employed, but the results do not concur in regards to the pairing symmetry. To investigate this further, thermal conductivity measurements were performed on single crystal X = Cu[N(CN)₂]Br and Cu(NCS)₂, both of which undergo superconducting transitions at T_c ≈ 11.5 K and 9.5 K, respectively. In both samples, a relatively large peak in the temperature dependence appears just below T_c which is due to the opening of a superconducting gap and is indicative of high-quality samples. Furthermore, at zero magnetic field, the phonon contribution dominates in both samples at low temperatures (down to 150 mK), as evidenced by a T³ behavior, with no significant remnant electronic contribution in X = Cu[N(CN)₂]Br and only a small contribution in X = Cu(NCS)₂. Interestingly, when a magnetic field of μ_oH = 14 T is applied parallel to the conducting layers, the thermal conductivity is smaller in X = Cu[N(CN)₂]Br compared to the zero field data, but is significantly larger in X = Cu(NCS)₂. Our results demonstrate the importance of electron-phonon scattering in these Q2D superconductors.