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The quantum anomalous Hall effect is gradually quantized from the integer to the Anderson localized Skyrmion and Meron diagrams as the impurity intensity increases. As the impurity intensity increases, the valence band and the conduction band carrying the integer topological charge (Skyrmion) split into four Meron ADs with half-integer topology charges; after the impurity intensity exceeds the critical value, the B/C Meron carrying the opposite topological charge occurs. Exchange, so that the total topological charge below the energy gap (ie, the total Bely curvature) is zero, and the system can become Anderson insulators.
Professor Qiao Zhenhua, a professor at the University of Science and Technology of China, collaborated with domestic and foreign counterparts to make a series of progresses in theoretical research on topological quantum states of two-dimensional systems. Relevant results were published in Nature-Nanotechnology, Physical Review Letters and Progress Report on Physics.
The quantum anomalous Hall effect (ie, the quantum Hall effect under zero magnetic field conditions) has attracted wide attention in the field of condensed matter physics and materials science since the discovery of graphene and topological insulators, and has also made tremendous breakthroughs in experiments in recent years. One question that is opposite to how to make an integer quantized anomalous Hall effect is: How can the quantum anomalous Hall effect be destroyed and eventually become Anderson insulators in the presence of disorder/impurity? By systematically studying the characteristics of electron transport, Belle's curvature analysis, and localized length calculations, Qiao Zhenhua's team and collaborators discovered a novel quantum anomalous Hall effect in the case of spin inversion of impurities in Anderson's locality. The new physical mechanism of physics, that is, the beli curvature corresponding to the valence band and the conduction band is exchanged under the influence of impurities, thereby realizing the localization of quantum anomalous Hall effect. The results were published in the July 29th Physical Review Letters [Phys. Rev. Lett. 117, 056802 (2016)].
Due to its unique linear Dirac dispersion relationship, graphene becomes an ideal carrier for studying various topological quantum states. Graphene is predicted to achieve two-dimensional Z2 topological insulators and quantum anomalous Hall effects by inducing ferromagnetic or spin-orbit coupling through external regulation (atomic adsorption or coupled substrates). However, due to the extremely weak spin-orbit coupling induced by them, these two types of topological quantum states have not yet been experimentally implemented. In addition to electron spins, graphene also has the freedom of the valley KK'. By introducing different in-position energy in the two AB sub-lattices of single-layer graphene or by applying a vertical electric field in the double-layer graphene, the physical energy gap can be opened to realize the quantum valley Hall effect. However, this topological state is very sensitive to the configuration of the system edge, so it is difficult to achieve in experiments. When the applied vertical electric field changes with space, a topologically constrained one-dimensional zero-mode conduction state is formed in the vicinity of the electric field with zero strength. Although this topological state has been proposed theoretically in 2008, there has been no breakthrough due to experimental techniques. After two years of hard work, Qiao Zhenhua's research team cooperated with Professor Zhu Wei's research group at Penn State University to achieve this topologically constrained state on bilayer graphene. Due to impurities, the average free path of the electronic state can reach hundreds of nanometers, although no dissipative integer quantized conductance can be achieved. Under the influence of an external magnetic field, the influence of impurity scattering caused by the valley is greatly weakened, so that the conductance can be close to the limit of integer quantization. This discovery has greatly facilitated the development and application of all-electrically-controlled, non-dissipative Valley electronics devices. The results were published in Nature-Nanotechnology on August 29 [Nat. Nanotech. 10, 1038 (2016)].
Over the years, Qiao Zhenhua and collaborators in graphene-based two-dimensional material system topological quantum states (such as quantum anomaly Hall effect, quantum spin Hall effect, quantum valley Hall effect, and topologically constrained one-dimensional zero mode, etc.) Directions conducted a series of studies and was invited on May 13th to publish an article in the Physical Review Journal “Physics Progress Report†[Rep. Prog. Phys. 79, 066501 (2016)], systematically reviewing the various types of Comprehensive progress in the theoretical and experimental aspects of the topological state of dimensional materials. The first author of the work was Ren Yafei, a doctoral student in the Physics Department.
The above series of research has been funded by the Committee, the Chinese Academy of Sciences, the Ministry of Science and Technology, and the Ministry of Education. The China University of Science and Technology Supercomputing Center also provided crucial support for the successful completion of these tasks.
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