Fermi surface studies in the low- and high-Tc phase of the organic superconductor β-(BEDT-TTF)2I3

We report de Haas-van Alphen (dHvA) measurements of the high-Tc modification (Tc=7-8 K) and Shubnikov-de Haas results of the low-Tc modification (Tc∼1.4 K) of the organic superconductor β-(BEDT-TTF)2I3. In the high-Tc phase we find a single closed two-dimensional orbit with a dHvA frequency F0=(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 mc=(4.2±0.2)me. 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 ΔF is t perpendicularto /εF∼1/175. At the angles where the beating disappears, i.e., ΔF∼0, the oscillations have a harmonic content which is much larger than expected from the Lifshitz-Kosevich formula. In the low-Tc phase weak SdH oscillations with a frequency of FSdH∼110 T and an effective cyclotron mass mc∼1.0 me is found. These results suggest a possible reconstruction of the Fermi surface in the low-Tc phase. (orig.)