Observation of chiral surface excitons in a topological insulator Bi2Se3 H.-H. Kunga,1,2, A. P. Goyalb, D. L. Maslovb,1, X. Wanga,c, A. Leea, A. F. Kemperd, S.-W. Cheonga,c, and G. Blumberga,e,1 aDepartment of Physics & Astronomy, Rutgers University, Piscataway, NJ 08854; bDepartment of Physics, University of Florida, Gainesville, FL 32611; cRutgers Center for Emergent Materials, Rutgers University, Piscataway, NJ 08854; dDepartment of Physics, North Carolina State University, Raleigh, NC 27695; and eLaboratory of Chemical Physics, National Institute of Chemical Physics and Biophysics, 12618 Tallinn, Estonia Edited by Angel Rubio, Max Planck Institute for the Structure and Dynamics of Matter, Hamburg, Germany, and approved January 22, 2019 (received for review August 5, 2018) The protected electron states at the boundaries or on the surfaces troscopy to study the secondary emission from a 2D electronic of topological insulators (TIs) have been the subject of intense the- system of significant current interest: the surface state of a oretical and experimental investigations. Such states are enforced 3D topological insulator (TI), Bi2Se3. [Polarized PL was also by very strong spin–orbit interaction in solids composed of observed in Bi1:95In0:05Se3 crystals (SI Appendix, section 3)]. In heavy elements. Here, we study the composite particles—chiral 3D TIs, strong SOC and time-reversal symmetry collaborate to excitons—formed by the Coulomb attraction between electrons support topologically protected massless surface states (denoted and holes residing on the surface of an archetypical 3D TI, Bi2Se3. by SS1 in Fig. 1 A and B) with chiral spin-momentum texture Photoluminescence (PL) emission arising due to recombination of (19–23). Both angular-resolved photoemission (ARPES) data excitons in conventional semiconductors is usually unpolarized and first-principle calculations show that there are two more sur- because of scattering by phonons and other degrees of freedom face bands near the Brillouin zone center (Γ-point) in Bi2Se3 during exciton thermalization. On the contrary, we observe almost (17, 18, 24): (i) a high-energy unoccupied Dirac cone (SS2) and perfectly polarization-preserving PL emission from chiral excitons. (ii) fully occupied Rashba-like surface states (RSS). These bands We demonstrate that the chiral excitons can be optically oriented are depicted by red lines in Fig. 1A and enclosed in boxes in with circularly polarized light in a broad range of excitation ener- Fig. 1B. Due to strong SOC, all three surface bands exhibit gies, even when the latter deviate from the (apparent) optical band spin-momentum locking, which could lead to optical orienta- gap by hundreds of millielectronvolts, and that the orientation tion of single-electron spins and excitons (4). So far, most of remains preserved even at room temperature. Based on the depen- the research on TIs has been focused on spin dynamics and col- dences of the PL spectra on the energy and polarization of incident lective modes of Dirac fermions in SS1 (25–28), and far less is photons, we propose that chiral excitons are made from massive known about the properties of RSS and SS2. We excite interband holes and massless (Dirac) electrons, both with chiral spin textures transitions between surface states RSS and SS2 with circularly enforced by strong spin–orbit coupling. A theoretical model based polarized light and study polarization of PL emission in the on this proposal describes quantitatively the experimental obser- backscattering geometry, with light being incident normally on vations. The optical orientation of composite particles, the chiral the crystal surface. excitons, emerges as a general result of strong spin–orbit cou- pling in a 2D electron system. Our findings can potentially expand Significance applications of TIs in photonics and optoelectronics. We observe composite particles—chiral excitons—residing on exciton j topological insulator j photoluminescence spectroscopy j the surface of a topological insulator (TI), Bi2Se3. Unlike other optical orientation known excitons composed of massive quasiparticles, chiral excitons are the bound states of surface massless electrons pin–orbit coupling (SOC) plays a central role in spintronic and surface massive holes, both subject to strong spin–orbit Sand optoelectronic applications by allowing optical control coupling which locks their spins and momenta into chiral of spin excitations and detection with circularly polarized light, textures. Due to this unusual feature, chiral excitons emit in the absence of an external magnetic field (1–3). This effect circularly polarized secondary light (photoluminescence) that is also known as optical orientation, where nonequilibrium dis- conserves the polarization of incident light. This means that tribution of spin-polarized quasiparticles is optically created in the out-of-plane angular momentum of a chiral exciton is pre- semiconductors with strong SOC (4, 5). Detailed information served against scattering events during thermalization, thus on spin dynamics can be obtained by studying polarized pho- enabling optical orientation of carriers even at room temper- toluminescence (PL) (6–9). Typically, the degree of PL polar- ature. The discovery of chiral excitons adds to the potential of ization in semiconductors decreases rapidly as the excitation TIs as a platform for photonics and optoelectronics devices. photon energy deviates from the optical band gap or with heat- ing (4, 5, 10). Elaborative layer and strain engineering is often Author contributions: G.B. designed research; H.-H.K. and G.B. designed the experiments; required to lift spin degeneracy of the bulk bands to achieve X.W. and S.-W.C. grew the single crystals; H.-H.K., A.L., and G.B. acquired and analyzed the optics data; A.P.G. and D.L.M. developed the theoretical model of chiral excitons; a higher degree of PL polarization (11). In contrast, nearly A.F.K. performed calculations of the band structure; and H.-H.K., A.P.G., D.L.M., X.W., complete PL polarization, observed recently in transition metal A.L., A.F.K., S.-W.C., and G.B. contributed discussions and wrote the paper.y dichalcogenide (TMD) monolayers up to room temperature, was The authors declare no conflict of interest.y attributed to spin–orbit-mediated coupling between the spin and This article is a PNAS Direct Submission.y valley degrees of freedom (12–14). The reduced dimensionality Published under the PNAS license. y suppresses dielectric screening and restricts the number of scat- 1 To whom correspondence may be addressed. Email: [email protected], sean. tering channels, resulting in long-lived coherent 2D excitons (15). [email protected], or [email protected]fl.edu.y These results have attracted significant interest due to possible 2 Present address: Quantum Matter Institute, University of British Columbia, Vancouver, applications and also as an insight into the nature of many-body BC V6T 1Z4, Canada.y interactions in 2D electronic and photonic systems (3, 13, 16). This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. In this paper, we discuss a class of excitons which produce 1073/pnas.1813514116/-/DCSupplemental.y helicity-preserving PL. We use polarization-resolved PL spec- Published online February 20, 2019. 4006–4011 j PNAS j March 5, 2019 j vol. 116 j no. 10 www.pnas.org/cgi/doi/10.1073/pnas.1813514116 Downloaded by guest on October 1, 2021 AB CD E F Fig. 1. (A) The electronic band structure near the Γ point as inferred from ARPES measurements (17, 18). The Rashba surface states (RSS) and the unoccupied topological surface states (SS2) are depicted by thick red lines, with the in-plane spin orientations denoted by and ⊗. The low-energy surface states (SS1) near the Fermi energy (EF ) are depicted by thin red lines. The bulk bands (which do not contribute to circularly polarized PL) are shown in gray. (B) Calculated band structure along the Γ–K cut in the Brillouin zone of the hexagonal lattice, projected onto the top quintu- ple layer for Jz = 1=2. The blue squares highlight the three surface bands (SI Appendix, section 2). (C) The PL spectra measured with right-circularly polarized 2.7-eV excitation at 15 K. Right- and left-circularly polarized PL signals are designated by IRR(!, T) in red and IRL(!, T) in green, respectively. The black line shows unpolarized PL background, f(!, T). The intensity in the 1.8- to 2.6-eV range is multiplied by factor of 3 for clarity. (D) The SCIENCES dispersion relation of noninteracting electron–hole pairs for possible transitions from RSS to SS2, with zero momentum transfer. The only transition APPLIED PHYSICAL consistent with the excitation energy used in this study is shown in blue; others are shown in gray. With finite interaction, the excitonic bound states form below the band minimum and are denoted by the total angular momenta of the electron–hole pairs, Jz.(E) The integrated polarized PL inten- R 2:5 eV sity, 2:0 [IRR(!, T) − IRL(!, T)]d! (shown as the shaded blue area in C), vs. excitation energy measured at 15 K. (F) The depolarization ratio r(T) ≡ IRL(!,T)−f(!,T) is plotted as a function of (!0), with T ≈ 15 K. The red line is a linear extrapolation to r(T) = 0, suggesting a minimum excitation threshold IRR(!,T)−f(!,T) energy of 2.5 eV. Experimental Results If relaxation processes of these states preserve their angular Polarized PL. Fig. 1C depicts the intensities of right- and left- momenta, the secondary photons are emitted with the same circularly polarized PL signals excited by right-circularly polar- polarization as that of the excitation one. For reasons that will ized light, IRR(!, T ) and IRL(!, T ), respectively, where ! is the become clear later on, we denote these states as jJz = 1i and energy of emitted photons and T is temperature.