Attractive Coulomb interaction, temperature-dependent hybridization, and natural circular dichroism in 1T-TiSe2

We study the electronic structure of single crystal 1T-TiSe2 using temperature (T)-dependent soft x-ray photoemission spectroscopy (PES) and natural circular dichroism (NCD) across the charge-density-wave (CDW) transition at T-cdw similar to 200 K. We investigate detailed changes in energy positions and widths of the Ti 2p and Se 3d core-level spectra across T-cdw. The Ti 2p-3d-resonant PES shows a clear Ti 3d character two-hole correlation satellite in the valence band. A Cini-Sawatzky analysis indicates an effective attractive two-hole Coulomb interaction energy U-dd =-1.3 eV. This attractive Coulomb energy is significantly larger than the known energy scale of excitonic correlations (< 100 meV) in 1T-TiSe2. On-resonant T-dependent measurements indicate that the strongly hybridized Ti 3d-Se 4p bands show increased hybridization in the CDWphase. On-resonant valence band spectra show a finite, possibly experimental geometry-related NCD of the states at and near the Fermi level which do not show T dependence across T-cdw. Given the presence of excitonic effects and the periodic lattice distortion, our results suggest that theoretical models need to take into account the attractive Coulomb interaction and hybridization changes for a complete understanding of the CDW in 1T-TiSe2.