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Jonathan A. SOBOTA - Stanford, USA

Electron and Lattice Dynamics of the Topological Insulator Bi2Se3 Studied by Ultrafast Photoemission Spectroscopy


Topological insulators are novel materials with a rich electronic structure characterized by the coexistence of insulating bulk states and robustly metallic surface states. Understanding the interplay of these electronic states, as well as their interaction with the underlying crystal lattice, is a prerequisite for technological application of these materials.

 

Here we use time- and angle- resolved photoemission spectroscopy (trARPES) to study the transient response of the topological insulator Bi2Se3 upon ultrafast photoexcitation. Our results fall into three categories : (1) By two-photon photoemission processes, we study the unoccupied electronic structure and resolve optical transitions into a second topological surface state. (2) By studying the electron relaxation dynamics, we identify the dominant scattering channels including interband surface-to-bulk scattering as well as intraband electron-phonon scattering. The temperature-dependence of the latter is measured and interpreted with a microscopic model of the scattering processes. (3) With sufficiently intense excitation, we drive coherent lattice motion which results in oscillatory modulations of both the bulk and surface bands. We find an additional frequency in the surface as compared to the bulk, which we interpret as a softening of the phonon near the crystal surface.