Electron dynamics in the spin-split surface states of semiconductors with strong spin-orbit interaction

This thesis investigates the excitation and dynamics of charge carriers in two narrow-gap semiconductors featuring spin-polarized surface states, which are promising candidates for spintronic applications. On the topological insulator Bi2Se3, we investigated the origin of helicity-dependent photocurrents by two-photon photoemission. On BiTeI, we studied the electron- and hole-dynamics in the surface state and the conduction band minimum (CBM), which both feature a giant Rashba-splitting, with time- and angleresolved photoemission spectroscopy. In the case of Bi2Se3, circular polarized pulses of 1.7 eV photon energy are used to excite electrons resonantly from the occupied topological surface state (TSS) to the unoccupied TSS. The excited electrons are photoemitted with s-polarized pulses of 4.7 eV, conserving the momentum distribution. The dichroic contrast created during the excitation is identified to be predominantly governed by the interaction of the light with the initial state, as corroborated by off-resonantly excited spectra, model calculations performed by our collaboration partners, and time-resolved data. Anti-symmetric patterns, as expected from the commonly assumed coupling of the photon angular momentum to the parallel component of the electron spin, can only be observed for electrons stemming from the CBM, but not from the TSS. The TSS, in contrast, shows clear three-fold symmetric patterns. Residual asymmetries, probably originating from experimental inaccuracies, are large enough to explain the helicity-dependent photocurrents observed in previous transport measurements. On BiTeI, the electronic system is probed with 6.2 eV photon energy, giving access to the carriers around the Fermi level. We confirm the reported electronic structure of the Te-terminated surface and find only minor influences on the electronic structure by the dopants manganese and vanadium. We observe a complex interplay of surface and bulk dynamics after photoexcitation with pulses of 1.5 eV photon energy. Electronelectron scattering is found to be effective across all subsystems. The dynamics above the Fermi level are governed by bulk interactions. Heat is dissipated from the hot electron gas by electron-phonon coupling. The long thermalization times between electrons and lattice suggest a phonon bottleneck, created by long lifetimes of optical phonons. The dynamics below the Fermi level at binding energies up to 50 meV are sped up by drift currents induced by the positively charged surface. Within the Rashba-split surface state (RSS), we find a temperature-dependent coupling of the photoholes to the surface plasmon. Furthermore, we refute previous reports of ballistic spin-dependent transport upon excitation with circular polarized pump pulses