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

TL;DR

This study investigates how high pressure influences the electronic structure of a dimer-Mott insulator, revealing the emergence of charge degrees of freedom and potential implications for high-temperature superconductivity.

Contribution

It provides experimental evidence of pressure-induced changes in charge transfer and electronic state, highlighting the role of charge fluctuations in superconductivity in organic materials.

Findings

Intra- and interdimer charge transfer bands observed at ambient pressure.

Intradimer charge transfer shifts to lower energies with pressure.

System approaches a charge-ordered state under high pressure.

Abstract

To elucidate the pressure evolution of the electronic structure in an antiferromagnetic dimer-Mott (DM) insulator ${\beta}^{\prime}$-(BEDT-TTF)$_2$ICl$_2$, which exhibits superconductivity at 14.2 K under 8 GPa, we measured the polarized infrared (IR) optical spectra under high pressure. At ambient pressure, two characteristic bands due to intra- and interdimer charge transfers have been observed in the IR spectra, supporting that this salt is a typical half-filled DM insulator at ambient pressure. With increasing pressure, however, the intradimer charge transfer excitation shifts to much lower energies, indicating that the effective electronic state changes from half-filled to 3/4-filled as a result of weakening of dimerization. This implies that the system approaches a charge-ordered state under high pressure, in which charge degrees of freedom emerge as an important factor. The present results suggest that charge fluctuation inside of dimers plays an important role in the high-temperature superconductivity.