INVESTIGATIONS OF FERMI SURFACES AND CYCLOTRON EFFECTIVE MASSES OF ORGANIC SUPERCONDUCTORS BY SHUBNIKOV-DEHAAS (SDH)-MEASUREMENT AND DEHAAS-VANALPHEN (DHVA)-MEASUREMENTS

The Fermi surfaces (FS) and effective masses of the organic supercondu ctors (BEDO-TTF)(2)ReO4(H2O) (1) AND kappa-(BEDT-TTF)(2)I-3 (2) were i nvestigated by SdH- and dHvA-measurements in magnetic fields up to 27T in the temperature range from 0.5K to 4.2K. In 1 two small closed poc kets (0.7% and 1.5% of the first Brillouin zone [FBZ]) are observed co rresponding very well with two cross-sectional areas of the FS obtaine d for a hole and electron pocket from tight binding calculations. In c ontrast to 1 in 2 two relatively large closed sections (13% and 85% of the FBZ) of the FS are observed, again cofirming the tight binding ca lculations. For 2 in magnetic fields above 12T the effective mass for the larger orbit, as obtained from the temperature dependence of the S dH-oscillation amplitudes, is magnetic field dependent as long as the field is arranged perpendicular to the conducting planes. In contrast, from dHvA-measurements - which were performed by turning the magnetic field by 27 degrees with respect to SdH-experiments - the observed ef fective mass is field independent (m=39m(0)). We suppose that the occ urrence of particles with fractional statistics at temperatures below 1K and in fields above 12T might be the reason for the observed field dependence of the effective mass in the SdH investigations.