Effects of Electron Correlations and Chemical Pressures on Superconductivity of $\beta^{\prime\prime}$-type Organic Compounds

TL;DR

This study explores how chemical modifications in organic superconductors influence their electronic states and superconducting transition temperatures, highlighting the role of structural parameters and electron correlations in pairing mechanisms.

Contribution

It demonstrates systematic relationships between chemical substitutions, structural changes, and electronic properties, proposing a charge-mediated pairing mechanism in organic superconductors.

Findings

Cell parameter changes correlate with $T_c$.

Charge disproportionation varies with $b$-axis length.

Band calculations confirm transfer integral variations.

Abstract

We investigate low-temperature electronic states of the series of organic conductors $\beta^{\prime\prime}$-(BEDT-TTF)$_4$[(H$_3$O)M(C$_2$O$_4$)$_3$]G, where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene, and 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 positive correlation with the superconducting transition temperature $T_{\rm c}$. Analyses of temperature and magnetic field dependences of the electrical resistance including the Shubnikov-de Haas oscillations elucidates that the variation of charge disproportionation, 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.