de Haas--van Alphen studies of the organic superconductors. alpha. -(ET) sub 2 (NH sub 4 )Hg(SCN) sub 4 and. kappa. -(ET) sub 2 Cu(NCS) sub 2 (with ET = bis(ethelenedithio)-tetrathiafulvalene)

We report measurements of the de Haas--van Alphen (dHvA) effect in the organic superconductors {alpha}-(ET){sub 2}(NH{sub 4})Hg(SCN){sub 4} and {kappa}-(ET){sub 2}Cu(NCS){sub 2} in fields up to 15 T and temperatures down to 0.45 K. In both compounds, the perfect 1/cos{theta} behavior of the dHvA frequency for angles {theta} of the applied field up to 70{degree} 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 {kappa}-(ET){sub 2}Cu(NCS){sub 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/cos{theta} 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.