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

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### 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.

### Author and article information

###### Journal
2016-01-19
2016-03-24
1601.04939