Charge-Transfer Complex $\kappa$-(BEST)$_2$Cu$_2$(CN)$_3$ Analogous to Organic Spin Liquid Candidate

We report the structural, electrical, and magnetic properties of the organic conductor $\kappa$-(BEST)$_2$Cu$_2$(CN)$_3$ (BEST: bis(ethylenediseleno)tetrathiafulvalene; abbreviated as $\kappa$-BEST-CN), which is isostructural with the quantum spin liquid candidate $\kappa$-(ET)$_2$Cu$_2$(CN)$_3$ (ET: bis(ethylenedithio)tetrathiafulvalene; abbreviated as $\kappa$-ET-CN). Resistivity measurements demonstrate that $\kappa$-BEST-CN exhibits semiconducting behavior, governed by the same conducting mechanism as $\kappa$-ET-CN. Under a pressure of ~0.1 GPa, $\kappa$-BEST-CN undergoes a superconducting transition with an onset temperature of ~4 K. From the comparison of the critical pressures of superconductivity between $\kappa$-ET-CN and $\kappa$-BEST-CN, $\kappa$-BEST-CN can be regarded as a chemically pressurized analogue of $\kappa$-ET-CN. Therefore, $\kappa$-BEST-CN, in which only the effective pressure changes without altering the anion structure, is considered a valuable reference material for elucidating the enigmatic properties observed in $\kappa$-ET-CN. Furthermore, the spin susceptibility of $\kappa$-BEST-CN is slightly larger than that of $\kappa$-ET-CN and shows weaker temperature dependence, which cannot be explained by the localized spin model. This behavior clarifies the anomalous magnetic properties of a system with frustration near the Mott transition, serving to stimulate future theoretical research.