Fig. 1

α-(BEDT-TTF)₂I₃ crystals, phase diagram, and infrared spectroscopy in a diamond anvil cell. a Crystal structure of α-(BEDT-TTF)₂I₃. b Measurement of the reflectivity in a diamond anvil cell (DAC). CuBe gasket have been used for a reference and the obtained reflectivity have been corrected for the reflectivity of the gasket. Ruby luminescence have been used as a pressure determination technique insitu at each temperature. c Phase diagram of α-(BEDT-TTF)₂I₃. Left scale shows the charge order transition temperature obtained from various methods (squares) and right scale represents charge gaps obtained via thermal activation and optics (circles). In this case they are not related with mean field theory; therefore, we scale the p = 0 data points to combine both. The Dirac state (correlated and massless Dirac fermions) related scaling also added to the phase diagram. There is a pressure range (between ~0.8 and ~1.3 GPa, shown lighter), where the accuracy of the optical measurements are decreased (due to the low-frequency limit) and the low-temperature upturn of the resistivity due to metal insulator transition and of different origin cannot be distinguished clearly. This low-temperature upturn has been identified as the Dirac state and extends to high-pressure regime,¹⁵ as also evident from our optical measurements (up to 4 GPa), while the charge order-Dirac state boundry can be identified with the correlated Dirac electrons. At ~2.3 GPa a very small-energy scale of these correlations has been identified via NMR experiments,¹⁸ while the current optical study also put forward the pressure dependence of the correlated Dirac state