Surface-phase transitions in the charge-density-wave systems 1T-TaSe₂ and 1T-TiSe₂

The goal of the present work is to investigate the electronic structure of the surfaces of the transition-metal dichalcogenides 1T-TaSe₂ and 1T-TiSe₂. In particular the modification of the electronic structure by enhanced interactions at the surface is studied. 1T-TaSe₂ shows a pronounced metal-to-insulator transition, which may be related to Mott physics and is not observable in bulk sensitive transport measurements. The first part of this work investigates the origin and the spatial extend of this transition. Time- and angle-resolved extreme ultraviolet photoelectron spectroscopy is used to determine the momentum-dependent electronic structure dynamics in the layered material 1T-TaSe₂. Extracted spectroscopic order parameters display a global two-time-scale dynamics indicating a quasi-instantaneous loss of the electronic orders and a subsequent coherent suppression of the lattice distortion on a time scale related to the frequency of the charge-density-wave amplitude mode. The fast reaction of the system reveals electronic interplay as the driving force to this phase transition. Moreover, the depth dependency of the charge-density wave in 1T-TaSe₂ at low temperatures is studied by hard X-ray photoelectron spectroscopy. The additional charge- density-wave-induced splitting of the Ta-4f-core levels, which serves as an excellent order parameter for the strength of the charge-density wave, is investigated. The findings corroborate the idea of a surface confined metal-to-insulator transition triggered by an enhancement of the charge-density wave at the surface. The second part of the present work focuses on the modification of the electronic structure at the surface of 1T-TiSe₂ enforced by electron doping. Angle-resolved photoelectron spectroscopy is used to investigate the influence on the charge-density wave in this material in detail. At high doping levels the charge-density wave vanishes and the complete electronically decoupling of the uppermost layer is observed.