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IMAGE: The definition of χ in systems with S4 symmetry on the orange surface S. The system in (b) has C2,110 symmetry. view more
Credit: @Science China Press
Using the symmetries of the systems, people can define various topological invariants to describe different topological states. The topological materials can be accurately discovered by calculating the topological invariants. Recently, researchers found that irreducible representations and compatibility relationships can be used to determine whether a material is topological nontrivial/trivial insulator (satisfying the compatibility relations) or topological semimetal (violating the compatibility relations), which leads to a large number of topological materials predicted by theoretical calculations. However, Weyl semimetals go beyond this paradigm because the existence of Weyl fermions does not need any symmetry protections (except for lattice translation symmetries). At present, people usually take a ve