Recently, Hybrid Order Topological Insulators in a Phononic journal Hybrid Order Topological Insulators in a Phononic journal Crystal (mixed order topological insulators in phononic crystals). Among them, South China University of Technology is the first signing unit of this paper, Yang Yating PhD student and Professor Lu Jiuyang from School of Physics and Optoelectronics are the co-first authors of this paper, and Professor Huang Xueqin, Associate Professor Deng Weiyin and Professor Liu Zhengyou from Wuhan University are the co-corresponding authors.

Screenshot of the paper website

  The study of topological states is a hot topic in condensed matter physics and material science in recent years. At present, many topological states have been realized, such as traditional first-order topological insulators, higher-order topological insulators and corresponding topological semi-metals. For a topological state, a single topological invariant is usually used to describe its topological properties. A natural question is: can many different topological phases, especially of different order, be implemented in the same system?

  This work answers the question positively. We have realized a new type of topological insulator in a two-layer phononic crystal, that is, a mixed-order topological insulator, in which there exists a one-dimensional bandgap free helical edge state in the band gap of the monopole mode, which originates from the first-order non-banal topology. There exists zero-dimensional angular states in the band gap of dipole mode, which originate from the second-order nonbanal topology. Figure (a) shows the protocell of a phononic crystal. The corresponding physical bands are shown in Figure (b), where the purple and cyan regions correspond to the band gaps formed by the monopole and dipole modes, respectively. Our theory and experiment confirm that the helical edge state is in the band gap of the unipolar mode, as shown in FIG. (c). The zero-dimensional angular state exists in the band gap of the dipole mode, as shown in Figure (d).

Figure: Acoustic mixed order topological insulator

  The acoustic team at the Artificial Microstructure Physics Laboratory of the School of Physics and Opto-electronics focuses on topological physics and novel transport operations in classical waves. He has published many articles in journals such as Nature Physics, Nature Materials, Nature Communications and Physical Review Letters with south China University of Technology as the first unit. This work was supported by the National Natural Science Foundation of China, the National Major Scientific Research Program of China, the Natural Science Foundation of Guangdong Province and the Innovation and Entrepreneurship Research Team Program of Guangdong Province.