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 GammaA 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 non-parabolic 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 3-dimensional wavevector. The experimental results demonstrate the absence of the Te 4pz* Fermi surface pocket at the Gamma point and significant self-energy renormalization of the valence band dispersions. Photoemission calculations based on a novel Bloch waves formalism within the one-step theory reveal limitations of understanding photoemission from layered materials such as TiTe2 in terms of direct transitions.