Berlin 2015 – wissenschaftliches Programm
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TT: Fachverband Tiefe Temperaturen
TT 77: Topological Insulators: Structure and Electronic Structure (jointly with HL, DS, MA, O)
TT 77.2: Vortrag
Mittwoch, 18. März 2015, 15:15–15:30, ER 270
Ultrafast currents at the surface of the topological insulator Bi2Se3 — •Lukas Braun1, Luca Perfetti2, Gregor Mussler3, Markus Münzenberg4, Martin Wolf1, and Tobias Kampfrath1 — 1Fritz-Haber-Institut Berlin (MPG) — 2Ecole Polytechnique Palaiseau — 3Forschungszentrum Jülich — 4Universität Greifswald
Optical excitation of topological insulators (TIs) can launch electron currents along the TI surface whose direction can be controlled by varying the polarization of the driving light [J. W. McIver et al., Nat. Nanotech. 7, 96]. So far, photocurrents have been detected with a time resolution from DC to picoseconds [C. W. Luo et al., Adv. Opt. Mat. 1, 804]. Since electrons moving through a solid typically undergo scattering on a 100fs time scale, it is highly desirable to generate and detect TI photocurrents with femtosecond time resolution in a contact-free manner. For this purpose, we excite n-doped Bi2Se3 (Fermi energy at 300meV) crystals with a femtosecond laser pulse (10fs, 1.55eV). The resulting photocurrent gives rise to the emission of a broadband terahertz (THz) electromagnetic pulse (1 to 20THz) whose transient electric field is detected by means of electro-optic sampling. We present a method that allows us to extract the transient current j(t) from the measured field E(t). The AC photocurrents are found to be dominated by shift currents along the surface and photo-Dember injection currents into the bulk. We finally discuss the origin of j(t) and implications for the dynamics of photoexcited TI electrons.