title: Collective excitations on a surface of topological insulator.
abstract: We study collective excitations in a helical electron liquid on a surface of three-dimensional topological insulator. Electron in helical liquid obeys Dirac-like equation for massless particles and direction of its spin is strictly determined by its momentum. Due to this spin-momentum locking, collective excitations in the system manifest themselves as coupled charge- and spin-density waves. We develop quantum field-theoretical description of spin-plasmons in helical liquid and study their properties and internal structure. Value of spin polarization arising in the system with excited spin-plasmons is calculated. We also consider the scattering of spin-plasmons on magnetic and nonmagnetic impurities and external potentials, and show that the scattering occurs mainly into two side lobes. Analogies with Dirac electron gas in graphene are discussed.PACS: 73.20.Mf; 73.22.Lp; 75.25.Dk.
authors.affiliation: Institute for Spectroscopy, Russian Academy of Sciences, Fizicheskaya 5, 142190, Troitsk, Moscow Region, Russia. lozovik@isan.troitsk.ru.
authors.forename: Dmitry K
authors.initials: DK
authors.lastname: Efimkin
authors.forename: Yurii E
authors.initials: YE
authors.lastname: Lozovik
authors.forename: Alexey A
authors.initials: AA
authors.lastname: Sokolik