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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.

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          Author and article information

          Journal
          2016-01-19
          2016-03-24
          10.1103/PhysRevB.93.195116
          1601.04939

          http://arxiv.org/licenses/nonexclusive-distrib/1.0/

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          13 pages, 17 figures, submitted to Phys. Rev. B
          cond-mat.str-el

          Condensed matter

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