De Haas-van Alphen effect in the organic superconductor -(BEDT-TTF) <Subscript>2 </Subscript>I <Subscript>3 </Subscript>

In the two-dimensional organic superconductor K-(BEDT-TTF)2I 3 de Haas-van Alphen oscillations were observed at magnetic fields above 5 T and temperatures between 0.4 and 2 K. We found two dHvA frequencies at 3.846 kT and 0.570 kT, which correspond to the cross-sectional areas of the Fermi surface expected from a tight-binding calculation. From the temperature dependence of the oscillation amplitudes the effective mass belonging to the larger orbit was found to be 3.80 mo. Precise measurements of the angular dependence of the dHvA frequency show no deviation from that expected for a cylindrical Fermi surface. The angular dependence of the amplitude including spin splitting zeroes can essentially be described by a two-dimensional Fermi surface. Certain systematical deviations, however, hint for a slight corrugation. PACS: 71.25.H; 74.70.K