Fermi surface studies in the low- and high- T c phase of the organic superconductor β -(BEDT-TTF) 2 I 3

We report de Haas-van Alphen (dHvA) measurements of the high- T c modification ( T c = 7 - 8 K) and Shubnikov-de Haas results of the low- T c modification (Tc ≈ 1:4 K) of the organic superconductor β -(BEDT-TTF) 2 I 3 . In the high- T c phase we find a single closed two-dimensional orbit with a dHvA frequency F 0 = (3815±10) T and a 1/cosθ dependence as expected for a quasi two-dimensional system (where θ is the angle between the magnetic field and the direction normal to the conducting planes). The effective cyclotron mass is m c = (4:2±0:2) m e . The dHvA signal shows a beating pattern caused by the three-dimensional warping of the Fermi cylinder. The inter-plane transfer integral estimated from the maximum beating frequency Δ F is t ⊥ /ε F ≈ 1/175. At the angles where the beating disappears, i.e., Δ F ≈ 0, the oscillations have a harmonic content which is much larger than expected from the Lifshitz-Kosevich formula. In the low- T c phase weak SdH oscillations with a frequency of F SdH ≈ 110 T and an effective cyclotron mass m c ≈ 1:0 m e is found. These results suggest a possible reconstruction of the Fermi surface in the low- T c phase.