Imaging phase separation near the Mott boundary of the correlated organic superconductors kappa-(BEDT-TTF)2X

Electronic phase separation consisting of the metallic and insulating domains with 50-100 microm in diameter is found in the organic Mott system kappa-(h8-BEDT-TTF)(1-x)(d8-BEDT-TTF)x]2CuN(CN)2]Br by means of scanning microregion infrared spectroscopy using the synchrotron radiation. The phase separation appears below the critical end temperature 35-40 K of the first-order Mott transition. The observation of the macroscopic size of the domains indicates a different class of the intrinsic electronic inhomogeneity from the nanoscale one reported in the inorganic Mott systems such as high-Tc copper and manganese oxides.