Forbidden orbits in the magnetic breakdown regime of κ-(BEDT-TTF)2Cu(NCS)2

Abstract We report on de Haas–van Alphen measurements on the quasi two-dimensional organic superconductor κ -(BEDT-TTF) 2 Cu(NCS) 2 in magnetic fields up to 27 T. Above 16 T where magnetic breakdown occurs we observe besides F α (closed orbit) and F β (breakdown orbit) also the difference frequency F β − α which cannot be explained by the semiclassical theory of magnetic breakdown. We will discuss the experimental results, also the observation of unusual effective electron masses, in the context of chemical potential oscillations.