Periodic orbit resonances in κ-(ET)2I3

We have observed periodic orbit resonances in the normal state of the organic superconductor κ-(ET) 2 I 3 . Frequency dependent studies allow for a determination of the cyclotron mass, which can then be compared with the effective mass obtained by other techniques. Tilting the sample in the applied magnetic field enhances the resonance harmonics and enables investigation of the warping of the nearly cylindrical Fermi surface; our findings confirm recent theoretical predictions for quasi-two-dimensional conductors.