Serendipity of a topological nontrivial band gap in the 2D borophene subunit lattice with broken mirror symmetry

Topological semimetal state with triply degenerate nodal points in a stable Cu2Te structure

Cu2Te is commonly used as the backside contact of CdTe-based solar cells. We predict a stable topological semimetal structure of Cu2Te(R3m) with triply degenerate nodal points near the Fermi energy. Triply degenerate nodal points are formed by the band crossing between two states with angular momentum j equal to 3/2 and 1/2 along the unique C3 axis. The anisotropic strain breaking C3 symmetry opens the energy gap, and transforms semimetal Cu2Te(R3m) into a topological insulator. It provides strong evidence for understanding the unconventional large linear magnetoresistance in Cu2-xTe. The band crossing of Cu2Te(R3m) strongly depends on the orbital on-site energy difference and the SOC strength. Crystal structures with the space group R3m (no. 160) are a good platform to obtain topological semimetals with triply degenerate nodal points. Compounds X2Y (X = Cu, Ag, Au, Y = O, S, Se, Te) except for Au2S and Cu2O are topological semimetals with triply degenerate nodal points around the Fermi energy.