Synthetic Metals, 55-57 (1993) 2891-2898 2891 DE HAAS - VAN ALPHEN STUDIES AND FERMI SURFACE PROPERTIES OF ORGANIC SUPERCONDUCTORS (ET)_2X J. WOSNITZA Physikalisches Institut, Universit~it Karlsruhe, W-7500 Karlsruhe (F.R.G.) G. W. CRABTREE, J. M. WILLIAMS, H. H. WANG, K. D. CARLSON, and U. GEISER Materials Science and Chemistry Division, Argonne National Laboratory, Argonne, II 60439 (U.S.A.) ABSTRACT de Haas - van Alphen (dHvA) measurements of organic superconductors (ET)2X, where ET stands for bis(ethylene)dithiofuivalene (or BEDT-TTF) and X = IBr 2, (NH4)Hg(SCN) 4 and Cu(NCS)2 are reported. The strong two-dimensionality of the Fermi surface (FS) is clearly seen by the perfect 1/cos(®)-behavior of the dHvA frequency. The distinctive kind of beating and the angular dependence of the dHvA signal in 13-(ET)2IBr 2 gives clear evidence for a lightly corrugated structure of the FS. Due to the nearly cylinder-shape of the FS the bare band structure effective mass, nab, also shows a 1/cos(®)-dependence which is responsible for spin splitting zeros at certain angles. At these points, where the fundamental amplitude of the dHvA signal is vanishing, na b could exactly be determined and by comparison with the independently measured cyclotron effective mass the electron-phonon coupling constant could be estimated. INTRODUCTION The largest number of organic superconductors with Tc's up to 13 K, charge transfer salts of the type (ET)2X, are characterised by their nearly two-dimensional (2D) electronic properties [1]. With the availability of very good single crystals during the last years the 2D electronic band structure and, in particular, the Fermi surface (FS) was investigated by numerous Shubnikov-de Haas (SdH) and de Haas-van Alphen (dHvA) experiments [2]. In this paper, we present results of our detailed dHvA-study of the angular dependence of the extremal area of the FS, the cyclotron effective mass and the dHvA amplitude of 13-(ET)2IBr 2, a- (ET)2(NH4)Hg(SCN)4 and ~:-(ET)2Cu(NCS)2. Previous measurements of the SdH effect in the former show contrary results concerning the FS structure [3, 4]. With our dHvA measurements we were able to investigate this discrepancy with a different method. We found beating of the dHvA signal over a wide angular range of the applied magnetic field reflecting the corrugated structure of the FS in this compound. In all investigated crystals we found a l/cos(O)-dependence of the extremal areas of the FS and consequently of the effective masses resulting in an unusual angular 0379-6779/93/$6.00 © 1993- Elsevier Sequoia. All fights reserved