| Among the quasi-2D transition-metal dichalcogenides (TMDC's), TiSe2 shows a special behaviour, with a transition at 202 K from a (1x1) to a distorted (2x2x2) charge density wave (CDW) phase, accompanied by an unusual behaviour of its resistivity with temperature. Though it has been studied for several decades now, the nature of the transition remains unclear, the most likely scenarios being a band Jahn-Teller mechanism and/or the onset of an excitonic insulator phase, which has never been directly evidenced so far. We give in this talk a strong evidence for the excitonic ground state in TiSe2, by direct comparison of theoretical calculations and experimental data. We present high resolution ARPES and STM/STS measurements of TiSe2 for various temperatures. In the RT phase the band structure consists in a Se 4p-derived valence band near the Gamma point and a slightly occupied Ti 3d-derived band near L. Upon entering the distorted phase, the spectral function shows backfolded bands with the CDW superperiodicity, and the band dispersions near the Fermi energy change. The backfolded bands carry an unusually large spectral weight, which will be discussed as a signature of the many-body interactions in the system. In STM the (2x2) CDW periodicity is clearly observed at the surface, and the differential conductivity spectra exhibit characteristic features near the Fermi energy. The experimental data are compared to theoretical calculations of the spectral function and density of states of an excitonic insulator. The agreement between theory and experiment is astonishingly good, thus giving a strong evidence of an excitonic ground state in TiSe2. |
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