A Family Group of Two-dimensional Node-Line Semimetals

Using evolutionary algorithm and first-principles calculations, we predict a family group of two-dimensional node-line semimetals MX (M=Pd, Pt; X=S, Se, Te), which has zig-zag type mono-layer structure in Pmm2 layer group. Band structure analysis reveals that node-line features are caused by band inversion and the inversion exists even in the absence of spin-orbital-coupling. Tests are carried out to confirm that the node-line loop is protected by crystal symmetry. This work extends our knowledge of node-line materials to two-dimensional cases, i.e., a group of metal-group VI compounds sharing the same lattice structure which has time reversion and crystal-mirror inversion symmetries.