Fermi-surface topology and the effects of intrinsic disorder in a class of charge-transfer salts containing magnetic ions:β″−(BEDT−TTF)4[(H3O)M(C2O4)3]Y(M=Ga, Cr, Fe;Y=C5H5N)

We report high-field magnetotransport measurements on ${\ensuremath{\beta}}^{\ensuremath{''}}\ensuremath{-}(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF}{)}_{4}[({\mathrm{H}}_{3}\mathrm{O})M({\mathrm{C}}_{2}{\mathrm{O}}_{4}{)}_{3}]Y,$ where $M=\mathrm{Ga},$ Cr, and Fe and $Y={\mathrm{C}}_{5}{\mathrm{H}}_{5}\mathrm{N}.$ We observe similar Shubnikov--de Haas oscillations in all compounds, attributable to four quasi-two-dimensional Fermi-surface pockets, the largest of which corresponds to a cross-sectional area $\ensuremath{\approx}8.5%$ of the Brillouin zone. The cross-sectional areas of the pockets are in agreement with the expectations for a compensated semimetal, and the corresponding effective masses are $\ensuremath{\sim}{m}_{\mathrm{e}},$ rather small compared to those of other BEDT-TTF salts. Apart from the case of the smallest Fermi-surface pocket, varying the M ion seems to have little effect on the overall Fermi-surface topology or on the effective masses. Despite the fact that all samples show quantum oscillations at low temperatures, indicative of Fermi liquid behavior, the sample and temperature dependence of the interlayer resistivity suggest that these systems are intrinsically inhomogeneous. It is thought that intrinsic tendency to disorder in the anions and/or the ethylene groups of the BEDT-TTF molecules leads to the coexistence of insulating and metallic states at low temperatures. A notional phase diagram is given for the general family of ${\ensuremath{\beta}}^{\ensuremath{''}}\ensuremath{-}(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF}{)}_{4}[({\mathrm{H}}_{3}\mathrm{O})M({\mathrm{C}}_{2}{\mathrm{O}}_{4}{)}_{3}]Y$ salts.