Fermiology of new charge-transfer salts, β″-(BEDT-TTF) 4[(H3O)M(C2O4)3]-solvent where M = Ga, Cr and Fe

We report high-field magnetotransport measurements on β-(BEDT-TTF) 4 [(H 3 O)M(C 2 O 4 ) 3 ]. solvent, where M=Ga 3+ , Cr 3+ and Fe 3+ and solvent=C 5 H 5 N. In spite of their differing transition metal-ions, M, the three compounds exhibit similar magnetic quantum oscillation spectra superimposed on a positive magnetoresistance. At least four independent quantum oscillation frequencies have been identified, corresponding to two different hole and electron pockets of the Fermi surface which follow the rules of a compensated metal. Observation of the small pockets could be the result of the Fermi surface reconstruction induced by a possible density wave. The effective masses are very similar for different samples and for different pockets range between m eff 0.5 - 1.1 m e whereas the Dingle temperatures varies between T D 1.4 - 4 K. At low temperature, the longitudinal magnetoresistance violates Kohler's rule, suggesting that the interlayer transport in these quasi-2D systems cannot be related to a single scattering time and that the disorder plays an important role.