Topological insulators are a fascinating group of materials. A spin-polarization occurs, as soon as an electric current flows in the material. MQC scientist Prof Dr Alexander Holleitner and his cooperation partners measured this now for the first time optically at room temperature. In particular, they succeeded to steer spin-polarized currents towards the edges by a circularly polarized light beam and to read-out the electron spin-polarization at the facets of the circuits.
About ten years ago, scientists discovered a group of materials called "topological insulators" with unusual electronic properties. The interior acts as an insulator, but the surface conducts electricity better than average. The group of the NIM physicist Professor Alexander Holleitner (weblink: www.nanoptronics.de) has succeeded to guide electrons with opposite magnetization, in short spin-polarization, towards the opposite edges of a topological insulator.
Key feature is that no external magnetic field is needed to generate this phenomenon. The opposite spin-polarization rather derives from an effect called spin-orbit-coupling. The direct coupling between the electron’s spin and the direction of the electron motion allows its manipulation. The physicists found this effect to be reversible. By inducing a certain magnetization with polarized light, they can control the electric current at the sample’s edges. Their results are presented in the latest issue of Nature Communications (Weblink: www.nature.com/articles/s41467-017-02671-1).
The best-known representatives of three-dimensional topological insulators are heavy metal alloys, such as bismuth selenide or bismuth telluride. Scientists assign the exceptional electronic properties to be a phenomenon of quantum physics: the so-called spin-Hall-effect. One observes that all electrons moving in the surface layers have a well-defined spin. In doing so, they differ "topologically" from electrons inside the materials. The direction of the surface currents is directly linked to the electron spin. In such spin-orbit materials, an electron with positive spin always flows in the opposite direction compared to an electron with negative spin.
Holleitner and colleagues now made the stunning discovery that this also holds for the material’s interior, if it is electrically conducting. When a current flows through the topological insulator, electrons with opposite spin move in opposite directions and accumulate at the topological edges of the material. The imbalance in the spin distribution results in a magnetization of the surface states.
Magnetic current flow: The spin-current
In conventional conductors, electric currents are always carried by electrons with an arbitrary spin-orientation. In topological insulators, however, the direct coupling between the electron’s spin and the direction of movement allows a particular control of the electrons without the necessity of a sophisticated magnetic field or magnetic materials.
"Such control of the electronic spin is the basic requirement for the realization of so-called spin-based electronics.", explains first author Paul Seifert, who designed and carried out the experiments. The scientists hope that this technology will be applied in the development of more powerful computers or the secure encryption of data.
Measurements with polarized light
Very small electric currents and their magnetization can be directly detected with polarized light. In the actual experiment, they contact a topological insulator between two electrodes and excite the material with a circularly polarized laser. By choosing the correct polarization, they can induce a magnetization in the material, as electrons with different spin can be excited selectively.
Through a circuit, the scientists are able to track how a spin-polarized current at the edges of the topological insulator changes when they change the polarization of the light. In addition, the scientists observed the local magnetization of the topological insulator to change the polarization of the reflected light. Thus, they were able to directly detect the magnetization or spin polarization generated by the current flow.
The experiments are funded by the Deutsche Forschungsgemeinschaft within DFG Projects 3324/8-1 of the SPP 1666 “topological insulator“ and the excellence cluster “Nanosystems Initiative Munich“ (NIM). The co-authors Dr. K. Vaklinova, Prof. K. Kern and Dr. M. Burghard work at the Max Planck Institute for Solid State Research in Stuttgart. Co-author Sergey Ganichev works at the Terahertz Center of the University of Regensburg.
Spin Hall photoconductance in a three-dimensional topological insulator at room temperature. Paul Seifert, Kristina Vaklinova, Sergey Ganichev, Klaus Kern, Marko Burghard und Alexander W. Holleitner. Nature Communications
Prof Dr Alexander Holleitner
Walter Schottky Institute and Physics-Department
Center for Nanotechnology and Nanomaterials
Technische Universität München
Am Coulombwall 4a
Helicity-dependent edge conductance. Picture: A Holleitner