Pressure-dependent optical investigations of α-(BEDT-TTF)₂I₃: Tuning charge order and narrow gap towards a Dirac semimetal

Infrared optical investigations of α-(BEDT-TTF)₂I₃ 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_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_CO by 8-9 K/kbar. The charge disproportionation Δρ diminishes by 0.017e/kbar and the optical gap Δ 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 electrons to the low-frequency optical conductivity, however, without being able to disentangle them unambiguously.