Investigation of interactions in two-dimensional electron gases with angular resolved photoemission spectroscopy

Abstract

In this thesis, angular resolved photoemission spectroscopy (ARPES) is used to study the electronic structure of different two-dimensional electron systems (2DES). This technique is very surface sensitive and the most direct method to probe the surface band structure of a system. In addition to the band dispersion also signatures from interactions can be detected with ARPES. A careful analysis of peak position and linewidth gives access to the complex self-energy Sigma, which describes the energy renormalization and the lifetime of the electrons due to many-body interactions. By using circularly polarized light it is possible to excite electrons selectively and to extract additional information about the electronic states in the surface. 2DESs exist typically either at interfaces or at surfaces. We put our focus on the surface states of topological insulators and investigate Bi2Te2Se and TiTe1.5Se0.5. The first is known to be a topological insulator with the Fermi energy located in the bulk band gap. A time-dependent doping behavior is observed and we show that the Coulomb interaction in this material is strongly dependent on the charge carrier density and not negligible for very low concentrations. TiTe1.5Se0.5 was predicted to be a topological insulator. We are able to present the surface band structure, but due to the fact that ARPES can only probe the occupied states it is impossible to see the predicted topological surface states. The third system under investigation is a monolayer of epitaxial graphene on SiC(0001) decorated by small amounts of Tl adatoms. ARPES measurements at very low temperature and very low impurity concentrations show that Tl adatoms not only give rise to electron doping but have also a large effect on the quasiparticle scattering rate. The adatoms introduce disorder and act on the graphene electronic structure both as Coulomb long-range scatterers as well as short-range scatterers with a delta-like potential. We show that also for charged impurities short-range scattering can play an important role and is not always negligible. By modeling the self-energy for long- and short-range scattering, we are able to extract a strong short-range scattering potential delta=-3.2 +/- 1 eV. A detailed understanding of the underlying scattering mechanisms in graphene is very important for the development of novel impurity-graphene-based electronics. ARPES experiments are also performed on the surface alloys on Ag(111). For these measurements, we use circularly polarized light and take advantage of the fact that the interaction between light and matter changes for different polarizations. Surface alloys on Ag(111) are formed by depositing 1/3 of a monolayer of bismuth or antimony on the clean Ag(111) surface, where every third Ag atom is replaced by such an alloy atom. These systems are known for their Rashba type spin splitting and the resulting chiral spin structure in the two-dimensional (2D) band structure. We show that circular dichroism (CD) in the angular distribution of the photoelectrons depends strongly on the experimental setup and the energy of the photons. Only under special circumstances it is possible to use CD to access the orbital angular momentum and - in systems with high spin-orbit coupling - maybe even the spin texture of the surface. Because of the final state effects, interpretation of CD data is quite complicated and has to be done very cautiously.