Formation of the excitonic insulator state in Ta2NiSe5

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More than half a century ago, theory predicts that electrons and holes will pair in proximity to a semiconductor-semimetal transition and even may form a macroscopically quantum coherent state at low temperatures, the so-called excitonic insulator (EI). But only recently there has been strong experimental evidence for such an equilibrium condensation phenomenon in a bulk material, mainly in systems with reduced electronic dimension. The most promising compound in this respect is quasi-one-dimensional (1D) Ta_2NiSe_5 (TNS), where the distinct flattening of the valence-band top observed in the angle-resolved photoemission spectroscopy below the structural transition temperature (T