It is well known that elementary particles such as fermions and bosons play an important role in our study of the universe. In recent years, nontrivial low-energy excitation in crystals has allowed physicists to have a deeper understanding of elementary particles in quantum field theory. However, so far, the research on topological materials mainly focuses on the electron-related Fermionic subsystem.

Recently, Associate Professor Xu Hu of the Department of Physics of Southern University of Science and Technology (SUSTech) worked with doctoral candidate Xia Bowen to make vital progress in phonon topology materials. Their paper, titled “Symmetry-Protected Ideal Type-II Weyl Phonons in CdTe” was published in top physics journal Physical Review Letters (IF = 9.227)




Recently, the research on topological phenomena in bosonic systems has been intensively studied such as the topological phenomena in artificial crystals and photonic crystals, but the research progress on topological phonon in terahertz band has been relatively slow. Physicists have successfully predicted and observed the low-energy quasi particle excitation of the Type-I and Type-II Weyl fermion, but the Type-II Weyl Phonon has not yet been theoretically predicted and experimentally observed in the phonon-related boson system.

This particular piece of research saw researchers study the phonon spectrum of CdTe materials by using the lattice dynamics method based on the density functional theory. The team then constructed a Wannier tight-binding Hamiltonian similar to the electronic system, so as to obtain information such as the topological edge state of the phonon system and the chirality of the topological phonon. They found that the Type-II Weyl phonon in CdTe exists at the boundary of the first Brillouin region, and the topological surface state is very long in the reciprocal space, which is very conducive to experimental observation.

By combining group theory and symmetry analysis, the researchers further proved that the Type-II Weyl boson in CdTe exists on the high symmetry line of Brillouin region boundary, and even if the inversion symmetry of system breaks. The results of this work not only provide valuable candidate materials for the experimental study of the Type-II Weyl phonon, but also provide a way to utilize the nontrivial phonon features in pratical applications.

Doctoral student Xia Bowen and Research Assistant Professor Wang Rui were co-first authors on the paper. Associate Professor Xu Hu was the correspondent author and SUSTech was the communication unit.

This work was supported by the National Natural Science Foundation of China, the Guangdong Natural Science Funds for Distinguished Young Scholars, the Fundamental Research Funds for the Central Universities of China, and the Science, Technology and Innovation Commission of Shenzhen Municipality.

Xia Bowen was a jointly trained doctoral candidate of SUSTech and Hong Kong University of Science and Technology. He was also a member of SUSTech’s Class of 2016, joining the research group of Associate Professor Xu Hu in his junior year before continuing in his group to complete his Ph.D. With the support of his doctoral supervisor, Xia Bowen has achieved a lot in magnetic topological & topological phononic materials. He has published five papers in top journals as the first author or co-first author, while collaborating in several high-level research papers that have been published in prestigious academic publications.

Original paper - https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.123.065501