title: Surface Collective Modes in the Topological Insulators Bi_{2}Se_{3} and Bi_{0.5}Sb_{1.5}Te_{3-x}Se_{x}.
abstract: We used low-energy, momentum-resolved inelastic electron scattering to study surface collective modes of the three-dimensional topological insulators Bi_{2}Se_{3} and Bi_{0.5}Sb_{1.5}Te_{3-x}Se_{x}. Our goal was to identify the "spin plasmon" predicted by Raghu and co-workers [Phys. Rev. Lett. 104, 116401 (2010)]. Instead, we found that the primary collective mode is a surface plasmon arising from the bulk, free carriers in these materials. This excitation dominates the spectral weight in the bosonic function of the surface χ^{"}(q,ω) at THz energy scales, and is the most likely origin of a quasiparticle dispersion kink observed in previous photoemission experiments. Our study suggests that the spin plasmon may mix with this other surface mode, calling for a more nuanced understanding of optical experiments in which the spin plasmon is reported to play a role.
authors.affiliation: Department of Physics and Seitz Materials Research Laboratory, University of Illinois, Urbana, Illinois 61801, USA.
authors.forename: A
authors.initials: A
authors.lastname: Kogar
authors.affiliation: Department of Physics and Seitz Materials Research Laboratory, University of Illinois, Urbana, Illinois 61801, USA.
authors.forename: S
authors.initials: S
authors.lastname: Vig
authors.affiliation: Department of Physics and Seitz Materials Research Laboratory, University of Illinois, Urbana, Illinois 61801, USA.
authors.forename: A
authors.initials: A
authors.lastname: Thaler
authors.affiliation: Department of Physics and Seitz Materials Research Laboratory, University of Illinois, Urbana, Illinois 61801, USA.
authors.forename: M H
authors.initials: MH
authors.lastname: Wong
authors.affiliation: Department of Physics and Seitz Materials Research Laboratory, University of Illinois, Urbana, Illinois 61801, USA.
authors.forename: Y
authors.initials: Y
authors.lastname: Xiao
authors.affiliation: Department of Physics and Seitz Materials Research Laboratory, University of Illinois, Urbana, Illinois 61801, USA.
authors.forename: D
authors.initials: D
authors.lastname: Reig-I-Plessis
authors.affiliation: Department of Physics, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Korea.
authors.forename: G Y
authors.initials: GY
authors.lastname: Cho
authors.affiliation: Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973, USA.
authors.forename: T
authors.initials: T
authors.lastname: Valla
authors.affiliation: Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973, USA.
authors.forename: Z
authors.initials: Z
authors.lastname: Pan
authors.affiliation: Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973, USA.
authors.forename: J
authors.initials: J
authors.lastname: Schneeloch
authors.affiliation: Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973, USA.
authors.forename: R
authors.initials: R
authors.lastname: Zhong
authors.affiliation: Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973, USA.
authors.forename: G D
authors.initials: GD
authors.lastname: Gu
authors.affiliation: Department of Physics and Seitz Materials Research Laboratory, University of Illinois, Urbana, Illinois 61801, USA.
authors.forename: T L
authors.initials: TL
authors.lastname: Hughes
authors.affiliation: Department of Physics and Seitz Materials Research Laboratory, University of Illinois, Urbana, Illinois 61801, USA.
authors.forename: G J
authors.initials: GJ
authors.lastname: MacDougall
authors.affiliation: Department of Physics and Seitz Materials Research Laboratory, University of Illinois, Urbana, Illinois 61801, USA.
authors.forename: T-C
authors.initials: TC
authors.lastname: Chiang
authors.affiliation: Department of Physics and Seitz Materials Research Laboratory, University of Illinois, Urbana, Illinois 61801, USA.
authors.forename: P
authors.initials: P
authors.lastname: Abbamonte