Fermi-surface reconstruction close to a pressure-induced metal-insulator transition

We report on a detailed investigation of the pressure-dependent structural and electronic properties close to the pres-sure-induced metal-insulator transition of the quasi-two-dimensional organic superconductor beta"-(BEDT-TTF)₂SF₅CH₂CF₂SO₃, where BEDT-TTF stand for bisethylenedithio-tetrathiafulvalene (or ET for short). Although the pressure-dependent hysteresis of the metal-insulator transition suggests a structural origin, no major crystallo-graphic modifications could be detected by neutron-scattering experiments. Shubnikov-de Haas (SdH) experiments, on the other hand, show that a reconstructed band structure precedes the phase transition to the insulating state. A new SdH frequency with a rather small effective mass appears.