Abstract
Topological insulators are characterized by Dirac-cone surface states with electron spins locked perpendicular to their linear momenta. Recent theoretical and experimental work implied that this specific spin texture should enable control of photoelectron spins by circularly polarized light. However, these reports questioned the so far accepted interpretation of spin-resolved photoelectron spectroscopy. We solve this puzzle and show that vacuum ultraviolet photons (50–70 eV) with linear or circular polarization indeed probe the initial-state spin texture of while circularly polarized 6-eV low-energy photons flip the electron spins out of plane and reverse their spin polarization, with its sign determined by the light helicity. Our photoemission calculations, taking into account the interplay between the varying probing depth, dipole-selection rules, and spin-dependent scattering effects involving initial and final states, explain these findings and reveal proper conditions for light-induced spin manipulation. Our results pave the way for future applications of topological insulators in optospintronic devices.
- Received 21 December 2012
DOI:https://doi.org/10.1103/PhysRevX.4.011046
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Published by the American Physical Society
Popular Summary
The electric conductivity at the surface of a three-dimensional topological insulator relies on spin-polarized electrons in a peculiar manner: Electrons in a left-moving current have their spins directed opposite to those in a right-moving one. For this condition to be fulfilled in all directions on a surface, the spins have to be confined to the surface plane. But how do we determine the orientation of the spins? The photoelectric effect, where electrons are excited to a higher-energy state (referred to as a “final state”) and then liberated from a solid surface by ultraviolet or x-ray light, has been used for this purpose. What is not clear is if, and how much, the light used in the measurement itself reorients the electron spins.
A recent theoretical work followed by a laser experiment using 6-eV photons reported that the use of circularly polarized light will always realign the electron spin with the direction of the incident light and control its orientation (parallel or antiparallel) through the direction of light polarization. In this combined experimental and theoretical paper, we report new findings, revealing that whether or not the spins of the photoexcited electrons are reoriented actually depends on the full symmetry properties of their final states.
We have studied the response of the spins of the photoelectrons from a prototypical topological insulator (bismuth selenide) in a wide range of photon energies. We have found that for typical photon energies employed in photoemission experiments (about 50 eV), the spins remain oriented within the surface plane. For very low photon energies, however, the photoemission process fully rotates the spin out of this plane, and its orientation is indeed controlled by the direction of circular polarization of the light.
Beyond the fundamental physics, there is also a practical question: Can light be used to manipulate the spin orientations of the surface currents of topological insulators toward the goal of light-controlled spintronic devices? The insights we have gained may prove crucial in answering this question.
