Observation of chiral surface excitons in a topological insulator Bi 2 Se 3

H. H. Kung, A. P. Goyal, D. L. Maslov, X. Wang, A. Lee, A. F. Kemper, S. W. Cheong, G. Blumberg

Research output: Contribution to journalArticlepeer-review

5 Scopus citations

Abstract

The protected electron states at the boundaries or on the surfaces of topological insulators (TIs) have been the subject of intense theoretical and experimental investigations. Such states are enforced by very strong spin–orbit interaction in solids composed of heavy elements. Here, we study the composite particles—chiral excitons—formed by the Coulomb attraction between electrons and holes residing on the surface of an archetypical 3D TI, Bi 2 Se 3 . Photoluminescence (PL) emission arising due to recombination of excitons in conventional semiconductors is usually unpolarized because of scattering by phonons and other degrees of freedom during exciton thermalization. On the contrary, we observe almost perfectly polarization-preserving PL emission from chiral excitons. We demonstrate that the chiral excitons can be optically oriented with circularly polarized light in a broad range of excitation energies, even when the latter deviate from the (apparent) optical band gap by hundreds of millielectronvolts, and that the orientation remains preserved even at room temperature. Based on the depen-dences of the PL spectra on the energy and polarization of incident photons, we propose that chiral excitons are made from massive holes and massless (Dirac) electrons, both with chiral spin textures enforced by strong spin–orbit coupling. A theoretical model based on this proposal describes quantitatively the experimental observations. The optical orientation of composite particles, the chiral excitons, emerges as a general result of strong spin–orbit coupling in a 2D electron system. Our findings can potentially expand applications of TIs in photonics and optoelectronics.

Original languageEnglish (US)
Pages (from-to)4006-4011
Number of pages6
JournalProceedings of the National Academy of Sciences of the United States of America
Volume116
Issue number10
DOIs
StatePublished - 2019

All Science Journal Classification (ASJC) codes

  • General

Keywords

  • Exciton
  • Optical orientation
  • Photoluminescence spectroscopy
  • Topological insulator

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