Recently, Ideal Type-II Weyl Phase and Topological Transition in Ideal Type-II Weyl Phase and Topological Transition in Physical Review Letters Phononic crystals ( Ideal Second-class Wael phases and topological Phase transitions in phononic Crystals). Among them, South China University of Technology is the first signing unit of the paper, Professor Huang Xueqin is the first author of the paper, associate professor Deng Weiyin, Associate Professor Lu Jiuyang and Professor Liu Zhengyou of Wuhan University are the corresponding authors of the paper.

  This study demonstrates the scientific problem of topological phase transition of Weyl semi-metals. The study of Weyl points and their topological properties has always been a hot topic in condensed matter physics. There are two types of Weyl points: one type of Weyl points with spotty Fermi surfaces, and the other type of Weyl points with strongly tilted conical dispersion and conical Fermi surfaces. This characteristic gives the second class Weyl semi-metals many unique properties, including anisotropic chiral anomalies and backhanded Landau levels. Both type I and type II Weyl points have been realized in condensed matter systems and artificial periodic structures, including photonic crystals and phononic crystals. However, these Weyl points are usually not at the same frequency, thus severely limiting further exploration of Weyl physics. To overcome this difficulty, scientists have recently proposed the concept of ideal Weyl points that are at the same frequency by symmetry correlation. Although the ideal class of Weyl points has been realized in photonic crystals, the ideal class of Weyl phases has not been realized yet. In addition, the Weyl points described by the three Pauli matrices are stable to perturbations, so the topological phase transitions of Weyl semi-metals are less studied.

Figure 1: Acoustical ideal Second class Weyl points and topological phase transitions

  In this study, a THREE-DIMENSIONAL Weyl phononic crystal was realized by 3D printing (FIG. 1 (a)). The phononic crystal is composed of layered triangular pillars arranged periodically, and the interlayer coupling is achieved by a partition with circular holes of different sizes. The structure has four ideal type II Weyl points in the momentum space (FIG. 1 (c) and (d)), which are protected by the symmetry of specular and time inversion and are at the same frequency. Meanwhile, the surface states of Fermi arcs exist on the surface of the Weyl phase. In addition, the phase transition from Weyl semi-metal to two different types of valley topological insulators was achieved by rotating the triangular pillars in the phononic crystal (FIG. 1 (b)). The Fermi ring surface states were found at the interfaces of the two different valley phases, where the dispersion isofars were closed. Anomalous shunt transport of Fermi ring surface states was further experimentally observed (FIG. 1 (e) and (f)).

  It is understood that the acoustic team in the Artificial Microstructure Physics Laboratory of the School of Physics and Opto-electronics mainly studies 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 is supported by the National Major Scientific Research Program of China (2018YFA0305800), the National Natural Science Foundation of China, the Pearl River Talents Program of Guangdong Province, the Outstanding Youth Fund of Guangdong Province, and the Basic Research Fund of the Central Universities.