Angular dependence of the cyclotron effective mass in organic superconductors

Abstract

We report measurements of the de Haas--van Alphen (dHvA) effect in the organic superconductors \ensuremath{\alpha}-(BEDT-TTF${)}_{2}$(${\mathrm{NH}}_{4}$)Hg(SCN${)}_{4}$ (${\mathit{T}}_{\mathit{c}}$\ensuremath{\approxeq}1 K) and \ensuremath{\kappa}-(BEDT-TTF${)}_{2}$Cu(NCS${)}_{2}$ (${\mathit{T}}_{\mathit{c}}$\ensuremath{\approxeq}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 ${\mathit{m}}_{\mathit{c}}$, 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.