FERMI-SURFACE STUDIES IN THE LOW-T-C AND HIGH-T-C PHASE OF THE ORGANIC SUPERCONDUCTOR BETA-(BEDT-TTF)(2)I-3

We report de Haas-van Alphen (dHvA) measurements of the high-T-c modif ication (T-c = 7 - 8 K) and Shubnikov-de Haas results of the low-T-c m odification (T-c approximate to 1.4 K) of the organic superconductor b eta-(BEDTTTF)(2)I-3. In the high-T-c phase we find a single closed two -dimensional orbit with a dHvA frequency F-0 = (3815 +/- 10)T and a 1/ cos Theta dependence as expected for a quasi two-dimensional system (w here 8 is the angle between the magnetic field and the direction norma l to the conducting planes). The effective cyclotron mass is m(c) = (4 .2 +/- 0.2)m(e). The dHvA signal shows a beating pattern caused by the three-dimensional warping of the Fermi cylinder. The inter-plane tran sfer integral estimated from the maximum beating frequency Delta F is t(perpendicular to)/epsilon(F) approximate to 1/175. At the angles whe re the beating disappears, i.e., Delta F approximate to 0, the oscilla tions have a harmonic content which is much larger than expected from the Lifshitz-Kosevich formula. In the low-T-c phase weak SdH oscillati ons with a frequency of F-SdH approximate to 110T and an effective cyc lotron mass m(c) approximate to 1.0 m(e) is found. These results sugge st a possible reconstruction of the Fermi surface in the low-T-c phase .