de Haas–van Alphen studies of the organic superconductors α-(ET${)}_2$(${\mathrm{NH}}_4$)Hg(SCN${)}_4$ and κ-(ET${)}_2$Cu(NCS${)}_2$ [with ET = bis(ethelenedithio)-tetrathiafulvalene]

We report measurements of the de Haas–van Alphen (dHvA) effect in the organic superconductors α-(ET${)}_2$(${\mathrm{NH}}_4$)Hg(SCN${)}_4$ and κ-(ET${)}_2$Cu(NCS${)}_2$ in fields up to 15 T and temperatures down to 0.45 K. In both compounds, the perfect 1/cosθ behavior of the dHvA frequency for angles θ of the applied field up to 70° from the normal to the conducting planes shows the strong two-dimensionality of the Fermi surface (FS). The angle-dependent magnetic-breakdown orbit found in κ-(ET${)}_2$Cu(NCS${)}_2$ hints at a small warping of the FS. The unusual angular dependence of the dHvA amplitude with up to 4 spin splitting zeros is quantitatively explainable by a 1/cosθ dependence of the bare band cyclotron-resonance effective mass. This distinctive behavior allows the separation of the mass enhancement due to electron-phonon coupling from the measured cyclotron-resonance effective mass and the estimation of the superconducting transition temperatures with the standard BCS formula, which are in qualitative agreement with the observed values.