Thursday, March 27, 2014

Manipulation of electrons using light

"Controlling electron spins by light: Researchers of HZB manipulate the electron spin at the surface of topological insulators systematically by light" 
2014-03-27 from apl. Prof. Dr. Oliver Rader [Rader (] [;sprache=en;typoid=3228]:
Topological insulators are considered a very promising material class for the development of future electronic devices. A research team at Helmholtz-Zentrum Berlin (HZB) has discovered, how light can be used to alter the physical properties of the electrons in these materials. Their results have just been published by the renowned journal "Physical Review X".
The material class of topological insulators has been discovered a few years ago and displays amazing properties: In their inside, they behave electrically insulating but at their surface they form metallic, conducting states. The electron spin, i. e., their intrinsic angular momentum, is playing a decisive role. Their sense of rotation is directly coupled to their direction of movement. This coupling leads not only to a high stability of the metallic property but also enables a particularly lossless electrical conduction. Topological insulators are, therefore, considered interesting and promising candidates for novel devices in information technology.
A particularly innovative approach is to try and influence the electron spin at the surface in such devices by light. HZB researcher Prof. Oliver Rader and his team have discovered by which means the spin at the surface of topological insulators can be altered. To this end, the researches performed experiments with light of various energies or wavelengths.

The wavelenght counts -
At the synchrotron radiation source BESSY II they investigated the topological insulator bismuth selenide (Bi2Se3) using a method called "spin-resolved photoelectron spectroscopy" – and gained astonishing insights: They found an astonishing difference depending on whether the electrons at the surface of the material are excited with circularly polarized light in the vacuum ultraviolet (50-70 electron volts, eV) or in the ultraviolet spectral range (6 eV). They could demonstrate that they can measure the spin of the electrons without changing it at higher energies which are typically used at synchtrotron light sources. "When excited at 50 eV, the emitted electros display the typical spin texture of topological insulators", Dr. Jaime Sánchez-Barriga, who conducted the experiments, explains. "The electron spins are in the surface aligned on a circle, similarly to a traffic sign for roundabout." This is the ground state of the electrons in the surface of topological insulators."
When excited by low-energy circularly polarized photons (6 eV), the spin of the electrons moved completely out of the surface plane. Above all, they adopted the spin orientation imposed by the right- or left-circularly polarized light. This means that the spin can be systematically manipulated – depending on the light that is used. The scientists can also explain the entirely different behavior at different energies which they attribute to symmetry properties. "Our result delivers important insight how lossless currents could be induced in topological insulators", Oliver Rader explains. "This is important for the development of so-called optospintronic devices which could enormously enhance the speed at which information is stored and processed."

DFG Priority Program -
Due to the high potential promised by topological insulators, the German Research Foundation DFG initiated the Priority Program "Topological Insulators: Materials – Fundamental Properties – Devices". Prof. Rader coordinates this program which aims at an improved understanding of the physics of the surface states in topological insulators.

Publication: Photoemission of Bi2Se3 with Circularly Polarized Light: Probe of Spin Polarization or Means for Spin Manipulation? Phys. Rev. X 4, 011046 – Published 24 March 2014; J. Sánchez-Barriga, A. Varykhalov, J. Braun, S.-Y. Xu, N. Alidoust, O. Kornilov, J. Minár, K. Hummer, G. Springholz, G. Bauer, R. Schumann, L. V. Yashina, H. Ebert, M. Z. Hasan, and O. Rader.

The picture shows the characteristic spin texture (arrows) in a topological insulator (bottom) and how it is either probed by circularly polarized light (top) or manipulated by it (middle). Picture: Rader/Sachez-Barriga/HZB

"Photoemission of Bi2Se3 with Circularly Polarized Light: Probe of Spin Polarization or Means for Spin Manipulation?"
Phys. Rev. X 4, 011046 – Published 24 March 2014
from: J. Sánchez-Barriga, A. Varykhalov, J. Braun, S.-Y. Xu, N. Alidoust, O. Kornilov, J. Minár, K. Hummer, G. Springholz, G. Bauer, R. Schumann, L. V. Yashina, H. Ebert, M. Z. Hasan, and O. Rader
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 Bi2Se3 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.

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.

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