Ultrafast Photoemission Studies of the Topological Insulator Bi2Se3

Download or read book Ultrafast Photoemission Studies of the Topological Insulator Bi2Se3 written by Jonathan A. Sobota and published by . This book was released on 2014 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Most macroscopic properties of a material (electrical, thermal, and optical) can be understood on the basis of its band structure and the dynamical behavior of its electrons. However, resolving electron dynamics directly is difficult using conventional techniques because the underlying interactions typically occur on a femtosecond timescale. Ultrafast photoemission spectroscopy is an experimental technique which can probe electronic band structure with femtosecond time resolution. This dissertation presents work using this technique to study dynamic phenomena in the topological insulator Bi2Se3, a fascinating material due to the coexistence of its insulating bulk and metallic surface electronic structures. It will demonstrate three types of novel measurements that can be performed with ultrafast photoemission spectroscopy which are unattainable by other methods: (1) By performing two-photon photoemission (2PPE) spectroscopy, it is possible to resolve unoccupied band structure above the Fermi level. In Bi2Se3 this reveals a second, topologically protected surface state. (2) Time-resolved 2PPE (tr2PPE) is used to record real-time "movies" of the electron relaxation which follows optical excitation. This allows for the identification of intra- and inter- band electron scattering pathways in the material. Temperature-dependent tr2PPE measurements are performed on the conduction band to help understand the role of electron-phonon coupling. A model of fundamental electron-phonon scattering processes is implemented to describe the results. (3) In a time- and angle- resolved photoemission spectroscopy (trARPES) experiment, intense optical excitation is used to drive coherent motion of A1g optical phonon modes in Bi2Se3. These modes lead to periodic binding energy shifts in the bulk conduction band and surface state. A slight red-shift of the phonon frequency is observed in the surface state, which is used to deduce a softening of the phonon due to the abrupt termination of the crystal at its surface.