Three-dimensional band structure of layered TiTe2:: Photoemission final-state effects

Three-dimensional band structure of unoccupied and occupied states of the prototype layered material TiTe2 is determined focusing on the Gamma A line of the Brillouin zone. Dispersions and lifetimes of the unoccupied states, acting as the final states in the photoemission process, are determined from a very-low-energy electron diffraction experiment supported by first-principles calculations based on a Bloch waves treatment of multiple scattering. The experimental unoccupied states of TiTe2 feature dramatic non-free-electron effects such as multiband composition and nonparabolic dispersions. The valence band layer-perpendicular dispersions are then determined from a photoemission experiment consistently interpreted on the basis of the experimental final states to achieve control over the three-dimensional wave vector. The experimental results demonstrate the absence of the Te 4p(z)(*) Fermi surface pocket at the Gamma point and significant self-energy renormalization of the valence band dispersions. Photoemission calculations based on a Bloch waves formalism within the one-step theory reveal limitations of understanding photoemission from layered materials such as TiTe2 in terms of direct transitions.