A material that is known for its potential as a magnetic topological insulator (MTI) could be put to different use. While investigating MnBi2Te4, a material composed of manganese, bismuth, and tellurium, a University of Chicago research team observed that the material’s magnetic properties changed quickly and easily in response to light. Based on this response, the team inferred that a laser could be used to encode information within the magnetic states of the material to optically store computational data. Using advanced spectroscopy techniques, the researchers further showed how the electrons in MnBi2Te4 compete between two opposing states — a topological state useful for encoding quantum information and a light-sensitive state useful for building energy-efficient optical storage devices. The team originally set out to explore MnBi2Te4’s capabilities as an MTI. Scientists believe that MnBi2Te4 should be able to host a quantum phenomenon known as “electron freeways,” in which an electric current flows in a 2D stream along the edges of the MTI. However, the material has been challenging to work with experimentally. “Our initial goal was to understand why it has been so hard to get these topological properties in MnBi2Te4,” professor Shuolong Yang said. “Why is the predicted physics not there?” Researchers in Yang Lab at the University of Chicago’s Pritzker School of Molecular Engineering have made unexpected progress toward developing a new optical memory that can quickly and energy-efficiently store and access computational data. Courtesy of Peter Allen/Pritzker School of Molecular Engineering, University of Chicago. The researchers used spectroscopy to visualize the behavior of the electrons within MnBi2Te4 in real time on ultrafast time scales. They combined time- and angle-resolved photoemission spectroscopy with time-resolved magneto-optical Kerr effect measurements, working with researchers at the University of Florida to observe the electromagnetism at the surface of the material. “This combination of techniques gave us direct information on not only how electrons were moving, but how their properties were coupled to light,” Yang said. The results of the analysis revealed why MnBi2Te4 did not show the behavior that is desired in topological materials. A quasi-2D electronic state in the material was competing with the topological state for electrons. Theoretical modeling was able to capture the initial quenching of a surface-rooted exchange gap within a factor of two, but overestimated the bulk demagnetization by one order of magnitude. This explained the sizable gap in the quasi-2D electronic state and the nonzero residual magnetization in even-layer MnBi2Te4. At the same time, it revealed the potential for efficient, light-induced demagnetization that would enable magnetism and topological orders to be manipulated for future topotronics. “There is a completely different type of surface electrons that replace the original topological surface electrons,” Yang said. “But it turns out that this quasi-2D state actually has a different, very useful property.” The second electronic state had a tight coupling between magnetism and external photons. While this condition is not useful for sensitive quantum data, it is essential for efficient optical memory. To further explore the potential application of MnBi2Te4 as a material for optically-controlled magnetic memory, Yang’s group will use a laser to manipulate the material’s properties experimentally. Yang believes that a better understanding of the balance between the two electron states on the surface of MnBi2Te4 could improve the material’s ability as an MTI and make it more effective for quantum data storage. “Perhaps we could learn to tune the balance between the original, theoretically predicted state and this new quasi-2D electronic state,” he said. “This might be possible by controlling our synthesis conditions.” The results of the research could lead to welcome advancements in optical memory. The team believes that an optical memory using MnBi2Te4 could be orders of magnitude more efficient than today’s typical electronic memory devices. “This really underscores how fundamental science can enable new ways of thinking about engineering applications very directly,” Yang said. “We started with the motivation to understand the molecular details of this material and ended up realizing it had previously undiscovered properties that make it very useful.” The research was published in Science Advances (www.science.org/doi/10.1126/sciadv.adn5696).
