Emergence of charge degrees of freedom under high pressure in an organic dimer-Mott insulator ${\beta}^{\prime}$-(BEDT-TTF)$_2$ICl$_2$

摘要

To elucidate the pressure evolution of the electronic structure in anantiferromagnetic dimer-Mott (DM) insulator${\beta}^{\prime}$-(BEDT-TTF)$_2$ICl$_2$, which exhibits superconductivity at14.2 K under 8 GPa, we measured the polarized infrared (IR) optical spectraunder high pressure. At ambient pressure, two characteristic bands due tointra- and interdimer charge transfers have been observed in the IR spectra,supporting that this salt is a typical half-filled DM insulator at ambientpressure. With increasing pressure, however, the intradimer charge transferexcitation shifts to much lower energies, indicating that the effectiveelectronic state changes from half-filled to 3/4-filled as a result ofweakening of dimerization. This implies that the system approaches acharge-ordered state under high pressure, in which charge degrees of freedomemerge as an important factor. The present results suggest that chargefluctuation inside of dimers plays an important role in the high-temperaturesuperconductivity.