부산대학교 도서관

We report high-field magnetotransport measurements on $\beta''$-(BEDT-TTF)4[ (H3O)M(C2O4)3] $\cdot$solvent, where M=Ga3+, Cr3+and Fe3+and solvent=C5H5N. 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 $m_{\rm eff} \approx 0.5-1.1~m_e$whereas the Dingle temperatures varies between $T_{\rm D} \approx 1.4-4$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. Key words. Magnetoresistance-quantum oscillations.