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

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, $T<T_{\mathrm{CO}}$, 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 $T_{\mathrm{CO}}$ by $8-9$~K/kbar. The charge disproportionation $\Delta \rho$ diminishes by $0.017e$/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.

13 pages, 17 figures, submitted to Phys. Rev. B

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