Ladungsträgerdynamik und Photoströme im Dirac-Kegel topologischer Isolatoren

This thesis presents investigations on the ultrafast dynamics of excited electrons and photocurrents in the topologically protected Dirac-cone-like surface state of threedimensional topological insulators. The focus is on the study of the prototype materials Sb2Te3 and Bi2Te3. Probably most important for possible future applications is the discovery, that THz radiation can drive electric currents in the Dirac cone of these materials. These currents consist of spin-polarized electrons which travel ballistically without losses over several hundreds of nanometers. Based on this, it might be possible to realize new light-wave driven electronics in the future, combining low power consumption and clock rates that exceed those of conventional semiconductor devices by at least three orders of magnitude. The experiments use time- and angle-resolved photoelectron spectroscopy (tr- ARPES) in combination with newly developed schemes for the transient direct excitation of electrons and photocurrents. Ultrashort mid infrared (MIR) and THz pump pulses are combined with femtosecond ultraviolet probe pulses for photoemission. Fundamentally new results could be obtained on three different topics. Two-photon photoemission experiments with visible pump pulses unambiguously show, for the first time, the existence of the linear dispersing Dirac cone in the bulk band gap of Sb2Te3 and Sb2Te2Se. Both materials are intrinsically p-doped, such that the Dirac cone was previously not accessible by conventional ARPES. Time-resolved experiments reveal that the decay of indirectly excited electrons in the Dirac-cone is dominated by scattering with electrons in the partially unoccupied valence band, as well as by transport into the bulk. Electron-phonon scattering is shown to play only a minor role. Mid infrared (MIR) pump pulses with photon energies in the range of the bulk band gap enable a direct optical transition from the occupied into the unoccupied part of the Dirac cone in Sb2Te3. This new excitation mechanism allows for the direct generation of an asymmetric electron distribution in momentum space. The timeresolved investigation of the redistribution of this asymmetry makes it possible to gain quantitative information about elastic momentum scattering, which is the limiting mechanism for charge transport. Elastic scattering times as long as 2.5 ps confirm theoretical predictions about considerable restrictions on momentum scattering in the surface state, due to its special spin texture. The results further show that the asymmetry depends on the crystal orientation and can be controlled with circularly polarized light of opposite helicity. VI Finally, a novel time- and angle-resolved photoemission experiment based on excitation in the THz range was developed and successfully demonstrated. For this purpose, the apparatus for photoelectron spectroscopy of our group in Marburg was combined with a laser setup of the group of Rubert Huber at the university of Regensburg which can produce intensive THz pump pulses. Femtosecond UV probe pulses allow for time-resolved photoemission with subcycle resolution. The new experiment makes it possible to drive electron currents in the topological surface state of Bi2Te3 with an electric field strength of up to 2,8 kV/cm at frequencies around 1 THz. The acceleration of the electrons in the sample was directly observed in momentum space with ARPES. The displacement of the Fermi circle of almost 10% of the Fermi wavevector leads to a spin-polarized current in the surface state with a current density of up to 2 A/cm. An experimentally challenging aspect of the THz excitation originates from the interaction of the electric field of the pump pulses with the photoemitted electrons in the vacuum, which can no longer be neglected as in previous tr-ARPES experiments. Methods were developed to correct the photoelectron spectra from additional shifts in kinetic energy and parallel momentum that arise from the acceleration and deceleration of the photoelectrons. At the same time, this so-called energy and momentum streaking makes it possible to sample the electric field at the surface in situ with high precision. The precise knowledge of the electric field transient makes it possible to extract characteristic times for elastic and inelastic scattering for the experiment. For this purpose, the time-resolved observation of the electron distribution in the surface state is compared with semi-classical calculations based on the Boltzmann-equation. In Bi2Te3, scattering times of 1 ps or higher could be derived. Thus, the revealed scattering times in this genuine time-resolved transport measurement are at least two orders of magnitude longer than in conventional materials. Combined with the inertia-free acceleration in the quasi-relativistic Dirac cone, this results in a ballistic mean free path of several hundred nanometers. Together, these results considerably improve our understanding of electron dynamics and strong-field interaction in novel solids. At the same time, they open the way towards all-coherent lightwave-driven electronic devices.