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

Abstract

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)₂I₃. 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 ΔF is t _⊥/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-T _c phase weak SdH oscillations with a frequency of F _SdH ≈ 110 T and an effective cyclotron mass m _c ≈ 1:0m _e is found. These results suggest a possible reconstruction of the Fermi surface in the low-T _c phase.