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arXiv · 1601.04939

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

R. Beyer·A. Dengl·T. Peterseim·S. Wackerow·T. Ivek·A. V. Pronin·D. Schweitzer·M. Dressel
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Abstract

Infrared optical investigations of $α$-(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 $Δρ$ diminishes by $0.017~e$/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 Dirac electrons to the low-frequency optical conductivity, however, without being able to disentangle them unambiguously.

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R. Beyer, A. Dengl, T. Peterseim, S. Wackerow, T. Ivek, A. V. Pronin, D. Schweitzer, M. Dressel. 2016-03-24. Pressure-dependent optical investigations of $α$-(BEDT-TTF)$_2$I$_3$: tuning charge order and narrow gap towards a Dirac semimetal. https://doi.org/10.1103/physrevb.93.195116

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