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              Researchers Show Various Twofold Symmetry-protected Weyl Points in Bosonic Systems

              Jul 21, 2020

              Nowadays, the study of the low-energy quasi-particles (the counterparts of the high-energy particles) in condensed matters is quite a hit. The Weyl fermions are very stable and have topological Chern number. To search for Weyl points (WPs) is a challenge due to the requirement of comprehensive calculations in general. Although the WPs do not need symmetry protections, additional symmetries can pin a WP at a fixed point.

              As we know, in spinful electronic systems, time-reversal (TR) symmetry makes that all Kramers pairs at the time-reversal-invariant momenta are WPs in chiral crystals. Then, we will ask if these symmetry-enforced Weyl points can emerge in bosonic systems. If yes, it will largely lower the difficulty to search for WPs.

              Very recently, the joint research group of Prof. WANG Zhijun from the Institute of Physics of the Chinese Academy of Sciences, together with Prof. FU Huahua from Huazhong University of Science and Technology, has performed the symmetry analysis of all 230 (single-valued) space groups.

              The researchers have given a complete list of the high-symmetry k-points, where the twofold WPs can be pinned and protected in bosonic systems. Their corresponding topological Chern numbers are given also. The results show that they can have Chern numbers of ±1, ±2 and ±4, respectively. These results can guide future experiments in the study of various WPs.

              In addition, at several k-points, only the two-dimensional irrep of WPs is allowed; namely, they are symmetry-enforced WPs, which are usually protected by nonsymmorphic crystal symmetry. Unlike the TR-enforced WPs in spinful electronic systems, where the WPs are usually buried in bulk states due to the weak strength of spin-orbit coupling, these nonsymmorphic-crystal-symmetry-enforced WPs in bosonic systems can be well exposed.

              Consequently, the associated surface arcs are long and robust, which can be easily probed in future experiments. The researchers have checked all the symmetry-enforced WPs in the calculations of phonon spectra of materials.

              This study entitled "Symmetry-enforced Weyl phonons" has been published on npj Computational Materials. It was supported by the National Natural Science Foundation of China and the Strategic Priority Research Program of the Chinese Academy of Sciences.


              WANG Zhijun

              Institute of Physics


              Symmetry-enforced Weyl phonons

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