We investigate low-temperature electronic states of the series of organic conductors ${\beta }^{\prime \prime }-{\left[\text{bis(ethylenedithio)tetrathiafulvalene}\right]}_4\left[\left({\mathrm{H}}_3\mathrm{O}\right)M{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_3\right]G$, where $M$ and $G$ represent trivalent metal ions and guest organic molecules, respectively. Our structural analyses reveal that the replacement of $M$ and $G$ give rise to systematic change in the cell parameters, especially in the $b$-axis length, which has a positive correlation with the superconducting transition temperature $T_{\mathrm{c}}$. Analysis of temperature and magnetic field dependences of the electrical resistance including the Shubnikov–de Haas oscillations elucidates that the variation of charge disproportionation, the effective mass, and the number of itinerant carriers can be systematically explained by the change of the $b$-axis length. The changes of the transfer integrals induced by stretching/compressing the $b$ axis are confirmed by the band calculation. We discuss that electron correlations in quarter-filled electronic bands lead to charge disproportionation and the possibility of a novel pairing mechanism of superconductivity mediated by charge degrees of freedom.

• Received 17 September 2019
• Revised 6 November 2019

DOI:https://doi.org/10.1103/PhysRevResearch.1.033184

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Published by the American Physical Society