Ab initio many-body effects in TiSe2 : a possible excitonic insulator scenario from GW band-shape renormalization

We present both theoretical ab-initio results within the Hedin's GW approximation and experimental angle-resolved photoemission and scanning tunneling spectroscopy measurements on TiSe2. With respect to the density-functional Kohn-Sham metallic picture, the many-body GW self-energy leads to a ≈0.2-eV band-gap insulator consistent with our STS spectra at 5 K. The highest valence and the lowest conduction bands are strongly renormalized, with a loss of k2 parabolic dispersion toward a k4 shape. In particular, GW moves the top of valence moved toward a circle of points away from Γ, arising in a Mexican hat shape commonly associated with an excitonic insulator. Our calculations are in good agreement with experiment.