Pressure-dependent optical investigations of $\alpha$-(BEDT-TTF)$_2$I$_3$: tuning charge order and narrow gap towards a Dirac semimetal

TL;DR

This study uses infrared optical spectroscopy under pressure to explore how charge order and electronic properties of $ ext{α-(BEDT-TTF)}_2 ext{I}_3$ evolve, revealing a linear suppression of charge order and a potential approach to a Dirac semimetal state.

Contribution

It provides detailed pressure-dependent optical data showing the suppression of charge order and the narrowing of the electronic gap, indicating tuning towards a Dirac semimetal phase.

Findings

Charge order transition temperature decreases linearly with pressure.

Charge disproportionation diminishes as pressure increases.

Optical gap narrows with pressure, approaching a potential Dirac semimetal state.

Abstract

Infrared optical investigations of $\alpha$-(BEDT-TTF)$_2$I$_3$ have been performed in the spectral range from 80 to 8000~cm$^{-1}$ down to temperatures as low as 10~K by applying hydrostatic pressure. In the metallic state, $T > 135$~K, we observe a 50\% increase in the Drude contribution as well as the mid-infrared band due to the growing intermolecular orbital overlap with pressure up to 11~kbar. In the ordered state, $T<T_{\rm CO}$, we extract how the electronic charge per molecule varies with temperature and pressure: Transport and optical studies demonstrate that charge order and metal-insulator transition coincide and consistently yield a linear decrease of the transition temperature $T_{\rm CO}$ by $8-9$~K/kbar. The charge disproportionation $\Delta\rho$ diminishes by $0.017~e$/kbar and the optical gap $\Delta$ between the bands decreases with pressure by -47~cm$^{-1}$/kbar. In our high-pressure and low-temperature experiments, we do observe contributions from the massive charge carriers as well as from massless Dirac electrons to the low-frequency optical conductivity, however, without being able to disentangle them unambiguously.