Photoelectron spin-flipping and texture manipulation in a topological insulator

TL;DR

This paper demonstrates that the spin polarization of photoelectrons emitted from topological insulators can be fully manipulated in three dimensions using light polarization, revealing new spin-dependent light-matter interaction effects.

Contribution

It introduces a novel method to control photoelectron spin polarization in all three dimensions via light polarization in topological insulators.

Findings

Spin polarization of photoelectrons can be fully manipulated in 3D.

Light polarization selection influences electron spin states.

Results suggest potential for tunable spin-polarized electron sources.

Abstract

Recently discovered materials called three-dimensional topological insulators constitute examples of symmetry protected topological states in the absence of applied magnetic fields and cryogenic temperatures. A hallmark characteristic of these non-magnetic bulk insulators is the protected metallic electronic states confined to the material's surfaces. Electrons in these surface states are spin polarized with their spins governed by their direction of travel (linear momentum), resulting in a helical spin texture in momentum space. Spin- and angle-resolved photoemission spectroscopy (spin-ARPES) has been the only tool capable of directly observing this central feature with simultaneous energy, momentum, and spin sensitivity. By using an innovative photoelectron spectrometer with a high-flux laser-based light source, we discovered another surprising property of these surface electrons which behave like Dirac fermions. We found that the spin polarization of the resulting photoelectrons can be fully manipulated in all three dimensions through selection of the light polarization. These surprising effects are due to the spin-dependent interaction of the helical Dirac fermions with light, which originates from the strong spin-orbit coupling in the material. Our results illustrate unusual scenarios in which the spin polarization of photoelectrons is completely different from the spin state of electrons in the originating initial states. The results also provide the basis for a novel source of highly spin-polarized electrons with tunable polarization in three dimensions.