Journal
JOURNAL OF ELECTRON SPECTROSCOPY AND RELATED PHENOMENA
Volume 195, Issue -, Pages 249-257Publisher
ELSEVIER SCIENCE BV
DOI: 10.1016/j.elspec.2014.01.005
Keywords
Topological insulator; Time- and angle-resolved photoemission; Two-photon photoemission; Electron-phonon scattering
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Funding
- Department of Energy, Office of Basic Energy Sciences, Division of Materials Science [DE-AC02-76SF00515]
- Stanford Graduate Fellowship
- Alexander-von-Humboldt Foundation
- M. Wolf
- Laboratory Directed Research and Development Program of Lawrence Berkeley National Laboratory under the U.S. Department of Energy [DE-AC02-05CH11231]
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We characterize the topological insulator Bi2Se3 using time- and angle-resolved photoemission spectroscopy. By employing two-photon photoemission, a complete picture of the unoccupied electronic structure from the Fermi level up to the vacuum level is obtained. We demonstrate that the unoccupied states host a second Dirac surface state which can be resonantly excited by 1.5 eV photons. We then study the ultrafast relaxation processes following optical excitation. We find that they culminate in a persistent non-equilibrium population of the first Dirac surface state, which is maintained by a metastable population of the bulk conduction band. Finally, we perform a temperature-dependent study of the electron-phonon scattering processes in the conduction band, and find the unexpected result that their rates decrease with increasing sample temperature. We develop a model of phonon emission and absorption from a population of electrons, and show that this counter-intuitive trend is the natural consequence of fundamental electron-phonon scattering processes. This analysis serves as an important reminder that the decay rates extracted by time-resolved photoemission are not in general equal to single electron scattering rates, but include contributions from filling and emptying processes from a continuum of states. Published by Elsevier B.V.
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