INVESTIGATIONS OF FERMI SURFACES AND EFFECTIVE MASSES OF THE ORGANIC SUPERCONDUCTORS (BEDO-TTF)(2)REO4(H2O) AND KAPPA-(BEDT-TTF)(2)I-3 BY SHUBNIKOV-DEHAAS AND DEHAAS-VANALPHEN MEASUREMENTS

The Fermi surfaces and effective masses of (BEDO-TTF)(2)ReO4(H-2 O) an d kappa-(BEDT-TTF)(2)I-3 were investigated by Shubnikov-de Haas (SdH) and de Haas-van Alphen (dHvA) measurements in magnetic fields up to 27 T in the temperature range from 0.5 K to 4.2 K. Two small closed pock ets (0.7% and 1.5% of the first Brillouin zone) are observed in (BEDO- TTF)(2)ReO4(H2O) corresponding very well with two cross-sectional area s of the Fermi surfaces obtained for a hole and an electron pocket fro m tight binding calculations. In contrast, in kappa-(BEDT-TTF)(2)I-3 t wo relatively large closed sections (13% and 85% of the first Brilloui n zone) of the Fermi surfaces are observed, again confirming the tight binding calculations. For kappa-(BEDT-TTF)(2)I-3 in magnetic fields a bove 12 T the effective mass for the larger orbit, as obtained from th e temperature dependence of the SdH-oscillation amplitudes, is magneti c field dependent as long as the field is arranged perpendicular to th e conducting planes (Theta = 0 degrees). In contrast, from dHvA measur ements - which were performed by turning the magnetic field by 27 degr ees with respect to the SdH experiments - the observed effective mass is field independent. We suppose that the occurrence of anyons at temp eratures below 1 K and in fields above 12 T might be the reason for th e observed field dependence of the effective mass in the SdH investiga tions under the special angle 0 degrees.