Angular dependence of the cyclotron effective mass in organic superconductors

We report measurements of the de Haas--van Alphen (dHvA) effect in the organic superconductors {α}-(BEDT-TTF)₂(NH₄)Hg(SCN)₄ (Tc{}1 K) and {κ}-(BEDT-TTF)₂Cu(NCS)₂ (Tc{}10 K) in fields up to 15 T and temperatures down to 0.45 K. For the first time, we determine the angular dependence of the cyclotron effective mass mc, and the orientation of up to four spin-splitting zeros of the dHvA signal. The angular dependence of the amplitude of the dHvA oscillations is quantitatively explained, and the mass enhancement due to electron-phonon coupling is estimated.