Regensburg 2016 – scientific programme

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O: Fachverband Oberflächenphysik

O 29: Topology- and Symmetry-Protected Materials

O 29.3: Talk

Tuesday, March 8, 2016, 11:15–11:30, S051

Generation of transient photocurrents in the topological surface state of Sb₂Te₃ bydirect optical excitation with mid-infrared pulses — •Johannes Reimann, Kenta Kuroda, Jens Güdde, and Ulrich Höfer — Fachbereich Physik und Zentrum für Materialwissenschaften, Philipps-Universität, D-35032 Marburg

We combine tunable mid-infrared (MIR) pump pulses with time- and angle-resolved two-photon photoemission (2PPE) to study the ultrafast electron dynamics of the topological surface state (TSS) of Sb₂Te₃. It is revealed that MIR pulses permit a direct excitation of the unoccupied TSS owing to an optical coupling across the Dirac point. This is in contrast to the delayed filling observed in previous 2PPE experiments on topological insulators with pump photon energies in the visible range. The novel optical coupling provokes asymmetric transient populations of the TSS at ±k_||, which mirrors a macroscopic photoexcited electric surface current. By observing the decay of the asymmetric population, we directly investigate the dynamics of the photocurrent in the time domain. We find a long equilibration time of τ_k^e=2.5 ps for the population at ±k_|| that shows no significant change for different sample temperatures of 80 K and 300 K. Considering a Debye temperature of θ_D = 162 K this result indicates that phonons play only a minor role for the momentum scattering. We suggest that scattering at surface defects is instead the limiting factor for the current lifetime.