  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.
