Investigations of Fermi Surfaces and Effective Masses of the Organic Superconductors (BEDO-TTF)$\text{}_{2}$ReO$\text{}_{4}$(H$\text{}_{2}$O) and κ-(BEDT-TTF)$\text{}_{2}$I$\text{}_{3}$ by Shubnikov-de Haas and de Haas-van Alphen Measurements

The Fermi surfaces and effective masses of (BEDO-TTF)$\text{}_{2}$ReO$\text{}_{4}$(H$\text{}_{2}$0) and κ-(BEDT-TTF)$\text{}_{2}$I$\text{}_{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 pockets (0.7% and 1.5% of the first Brillouin zone) are observed in (BEDO-TTF)$\text{}_{2}$ReO$\text{}_{4}$(H$\text{}_{2}$0) corresponding very well with two cross-sectional areas of the Fermi surfaces obtained for a hole and an electron pocket from tight binding calculations. In contrast, in κ-(BEDT-TTF)$\text{}_{2}$I$\text{}_{3}$ two relatively large closed sections (13% and 85% of the first Brillouin zone) of the Fermi surfaces are observed, again confirming the tight binding calculations. For κ-(BEDT-TTF)$\text{}_{2}$I$\text{}_{3}$ in magnetic fields above 12 T 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 (Θ = 0°). In contrast, from dHvA measurements - which were performed by turning the magnetic field by 27° with respect to the SdH experiments - the observed effective mass is field independent. We suppose that the occurrence of anyons at temperatures below 1 K and in fields above 12 T might be the reason for the observed field dependence of the effective mass in the SdH investigations under the special angle 0°.