The research group of Associate Professor Masaki Uchida of the Department of Physics, Tokyo Institute of Technology; Professor Masashi Kawasaki of Graduate School of Engineering, the University of Tokyo; and Yasujiro Taguchi of Group Director of the RIKEN Center for Emergent Matter Science, successfully demonstrated that the electronic states of two spatially separated surfaces are combined to exhibit the quantum Hall effect as a quantized three-dimensional orbital motion in a topological material, called a topological semimetal. This research group is the only group in the world to successfully produce a high-quality topological semimetal thin film that exhibits surface quantization conduction by a unique film formation method, where a special cap layer is deposited on the target thin film, and an additional temperature treatment is performed. According to Associate Professor Uchida, "We came up with a dual-gate experiment to investigate how the quantized state is realized in spatially separated surface orbits."
For this experiment, it is necessary to remove the cap layer after film formation and then deposit another gate insulating film. Through trial and error, the research group established this novel process. From this process they developed a dual-gate field-effect transistor device based on a high-quality topological semimetal thin film, and they conducted an experiment to independently control the carrier concentration on the upper and lower surfaces of the sample by an electric field. As a result, they succeeded in observing the quantum Hall effect related to the surface state by applying an electric field to the sample, where conduction in the three-dimensional bulk state was dominant, and by reducing the electron concentration.
Systematic electrical resistance measurements demonstrate that the background of the three-dimensional quantum effect is the Weyl orbit, in which spatially separated surface electronic states are linked by Weyl particles. In addition, as a result of sweeping the voltage of the top gate and back gate placed above and below the sample and mapping the change in the quantized all resistance value, it was clarified that a striped pattern is drawn instead of the checkerboard-like pattern, which is expected in the conventional cyclotron orbit. As a result, they found, for the first time, that the quantization transmission in a topological semimetal has a Weyl orbit spatial distribution in which the electronic states of the front and back surfaces of the sample are combined.
Associate Professor Uchida said, "Research has confirmed the existence of a chiral zero mode that connects surface orbits. However, this is a completely new electronic structure, and it is completely unknown what kind of conduction state is realized there. In the future, we would like to build the theory of quantum conduction based on this chiral zero mode before the rest of the world."
■ Weyl particle: A fermion obtained when the mass is zero in an equation that describes relativistic electrons. Proposed by German theoretical physicist Hermann Weyl in 1929.
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