Observation of Chiral Surface Excitons in a Topological Insulator Bi${}_2$Se${}_3$

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 three-dimensional topological insulator (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 meVs, and that the orientation remains preserved even at room temperature. Based on the dependences 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 such 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.

22 pages, 11 figures

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Cited by in corpus (11)

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• Tunable Electronic Properties and Large Rashba Splittings Found in Few-Layer Bi${}_2$Se${}_3$/PtSe${}_2$ Van der Waals Heterostructures
• Theory for all-optical responses in topological materials: the velocity gauge picture
• Exchange-Driven Intermixing of Bulk and Topological Surface State by Chiral Excitons in Bi2Se3
• Collective spin modes in Fermi liquids with spin-orbit coupling
• Electronic and Vibrational Excitations on the Surface of the Three-Dimensional Topological Insulator Bi${}_2$Te${}_{3-x}$Se${}_x$ (x = 0, 2, 3)
• Highly tunable optics across a topological transition in organic polymers
• Chiral Electronic Excitations in a Quasi-2D Rashba System BiTeI
• Local symmetry groups for arbitrary wavevectors
• Variational and field-theoretical approach to exciton-exciton interactions and biexcitons in semiconductors
• Control of Plasmons in Doped Topological Insulators via Basis Atoms