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

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Electronic phase separation consisting of the metallic and insulating domains with 50 -- 100 μm in diameter is found in the organic Mott system κ-[(h8-BEDT-TTF)₁₋ₓ(d8-BEDT-TTF)ₓ]₂Cu[N(CN)₂]Br by means of scanning micro-region 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 nano-scale one reported in the inorganic Mott systems such as High-Tc copper and manganese oxides.