Fermi surfaces of organic metals and superconductors (ET)2X

We review recent de Haas-van Alphen (dHvA) results of the quasi-two-dimensional organic metals α-(ET)2MHg(SeCN)4 (M = K and Tl). Up to 29 T we found field-independent effective cyclotron masses of (1.9–2.0) me for fields perpendicular to the ET planes. The observed spin-splitting zeros suggest, in comparison with the isostructural organic superconductor α-(ET)2NH4Hg(SCN)4, a reduced electron-phonon coupling in the non-superconducting salts. We further present dHvA and Shubnikov-de Haas (SdH) studies of the organic superconductor β-(ET)2I3 in the lowand high-Tc state indicate an appreciable influence of electron-electron interaction in this material. In the low-Tc state the Fermi surface seems to be completely different from the one mapped out in the high-Tc state.