Magnetic Field Dependent Effective Mass in the Organic Superconductor κ-(BEDT-TTF)$\text{}_{2}$I$\text{}_{3}$

κ-(BEDT-TTF)$\text{}_{2}$I$\text{}_{3}$ is an electronically extreme two-dimensional organic metal with a superconducting transition at around 4 K. In magnetic fields above 12 T the effective mass, as obtained from the temperature dependence of the amplitudes of Shubnikov-de Haas oscillations, is magnetic field dependent as long as the magnetic field is arranged perpendicular to the conducting planes. In contrast to this, by turning the magnetic field by 27° or even only 9° the observed effective mass (as obtained from Shubnikov-de Haas and de Haas-van Alphen measurements) is field independent. We suppose that the occurrence of anyons at temperatures below 1 K in fields above 12 T is the reason for the observed field dependence of the effective mass.