Pressure dependence of the Shubnikov-de Haas oscillation spectrum of $\beta''$ -(BEDT-TTF) 4(NH 4)[ Cr(C 2 O 4) 3] .DMF

The Shubnikov-de Haas (SdH) oscillation spectra of the $\beta''$ -(BEDT-TTF) 4(NH 4)[ Cr(C 2O 4) 3] .DMF organic metal have been studied in pulsed magnetic fields of up to either 36 T at ambient pressure or 50 T under hydrostatic pressures of up to 1 GPa. The ambient pressure SdH oscillation spectra can be accounted for by up to six fundamental frequencies which points to a rather complex Fermi surface (FS). A noticeable pressure-induced modification of the FS topology is evidenced since the number of frequencies observed in the spectra progressively decreases as the pressure increases. Above 0.8 GPa, only three compensated orbits are observed, as it is the case for several other isostructural salts of the same family at ambient pressure. Contrary to other organic metals, of which the FS can be regarded as a network of orbits, no frequency combinations are observed for the studied salt, likely due to high magnetic breakdown gap values or (and) high disorder level evidenced by Dingle temperatures as large as ≃7 K. Copyright EDP Sciences/Società Italiana di Fisica/Springer-Verlag 200671.18.+y Fermi surface: calculations and measurements; effective mass, g factor, 71.20.Rv Polymers and organic compounds, 72.20.My Galvanomagnetic and other magnetotransport effects,