Photoemission of bands above the Fermi level: The excitonic insulator phase transition in1T−TiSe2

High-resolution angle-resolved photoemission spectroscopy (ARPES) was used to investigate the transition metal dichalcogenide (TMC) $1T\ensuremath{-}{\mathrm{TiSe}}_{2}$ above and below the phase transition. We find that this system fulfills special conditions such as narrow band width and flat dispersion for bands within $5{\mathrm{k}}_{B}T$ of the Fermi energy. These prerequisites allow ARPES to observe energy dispersion of bands above ${E}_{F}$ without normalization procedures and a leading edge of the Fermi-Dirac distribution cutoff, which is considerably shifted to the unoccupied region with respect to ${E}_{F}.$ As a consequence we conclude that the $\mathrm{Ti}3d$ band is only thermally occupied at room temperature and considerably shifts towards the occupied range upon cooling. When passing the phase transition, the $\mathrm{Se}4p$ bands become backfolded due to new symmetry restrictions. The temperature behavior of the ARPES spectra can, in accordance to transport data, be explained as the occurrence of an excitonic phase suggested by Kohn [Phys. Rev. Lett. 19, 439 (1967)].