Collective excitations on a surface of topological insulator

TL;DR

This paper investigates collective charge and spin excitations, called spin-plasmons, on the surface of topological insulators, revealing their properties, internal structure, and scattering behavior in a helical electron liquid.

Contribution

It develops a quantum field-theoretical framework for spin-plasmons in helical liquids and analyzes their properties, scattering, and spin polarization, highlighting novel collective excitation behavior.

Findings

Spin-plasmons exhibit coupled charge- and spin-density waves.

Scattering mainly occurs into two side lobes.

Calculated spin polarization in excited states.

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 particless 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.