Electronic Correlations Among the Dirac Electrons in $\alpha$-(BEDT-TTF)$_2$I$_3$ Unveiled by High-Pressure Optical Spectroscopy

TL;DR

This study uses high-pressure optical spectroscopy to explore how electronic correlations among Dirac electrons in $ ext{(BEDT-TTF)}_2 ext{I}_3$ evolve from insulating to metallic states, revealing a pressure-tunable energy gap and coexistence of different Dirac fermions.

Contribution

It uncovers the pressure-dependent evolution of Dirac electrons and the emergence of correlated massive Dirac fermions in $ ext{(BEDT-TTF)}_2 ext{I}_3$ using infrared spectroscopy.

Findings

Observation of a pressure-induced transition from insulator to metal.

Detection of a pressure-tunable energy gap due to correlated massive Dirac fermions.

Evidence of coexistence of trivial and massless Dirac electrons in the metallic state.

Abstract

The charge-ordered insulator $\alpha$-(BEDT-TTF)$_2$I$_3$ gradually evolves to a metal when pressure is applied, and at low temperatures the electronic bands form tilted Dirac-like cones. A metallic state with a frequency-independent optical conductivity indicates the coexistence of the trivial and massless Dirac electrons in the system. Our infrared investigations also reveal that at the boundary between insulating and metallic states an energy gap opens due to correlated massive Dirac fermions, which is gradually suppressed when pressure increases.