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

We report on de Haas-van Alphen measurements on the quasi two-dimensional o rganic superconductor kappa-(BEDT-TTF)(2)Cu(NCS)(2) in magnetic fields up t o 27 T. Above 16 T where magnetic breakdown occurs we observe besides F-alp ha (closed orbit) and F-beta (breakdown orbit) also the difference frequenc y Fbeta-alpha which cannot be explained by the semiclassical theory of magn etic breakdown. We will discuss the experimental results, also the observat ion of unusual effective electron masses, in the context of chemical potent ial oscillations. (C) 1999 Elsevier Science B.V. All rights reserved.