THE FERMI SURFACES OF BETA-(BEDT-TTF)(2)X

We present de Haas-van Alphen (dHvA) measurements of the isostructural quasi-two-dimensional (quasi-2D) organic superconductors beta-(BEDT-T TF)(2)IBr2 and beta(H)-(BEDT-TTF)(2)I-3 with superconducting transitio n temperatures of T-c approximate to 2.5 K and T-c approximate to 7 K, respectively. For both salts almost the same dHvA frequency and a bea ting behavior of the dHvA signal are found which reflect the similarit y of the Fermi surface (FS) topology in both materials. The FS can be well described by a slightly corrugated quasi-2D cylinder. The corresp onding overlap integral t(perpendicular to) between the highly conduct ing organic layers could be determined with high precision. t(perpendi cular to) is larger for beta(H)-(BEDT-TTF)(2)I-3 although this salt ha s the larger interlayer distance. The effective cyclotron masses are v ery similar in both materials (m(c) approximate to 4m(e)). On the othe r hand, the observed spin-splitting zeros suggest a rather large diffe rence and large absolute values of the conduction electron g-factor wh ich seems to reflect the importance of electron-electron interactions in the beta-phase salts.