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 superconductors (BEDO-TTF)(2)ReO4(H2O) (1) AND kappa-(BEDT-TTF)(2)I-3 (2) were investigated 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 pockets (0.7% and 1.5% of the first Brillouin zone [FBZ]) are observed corresponding very well with two cross-sectional areas of the FS obtained for a hole and electron pocket from tight binding calculations. In contrast 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 calculations. For 2 in magnetic fields above 12T the effective mass for the larger orbit, as obtained from the temperature dependence of the SdH-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 effective mass is field independent (m*=39m(0)). We suppose that the occurrence 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.