  Topological semimetals have energy bands near the Fermi energy sticking
together at isolated points/lines/planes in the momentum space, which are often
accompanied by stable surface states and intriguing bulk topological responses.
Although it has been known that certain crystalline symmetries play an
important role in protecting band degeneracy, a general recipe for stabilizing
the degeneracy, especially in the presence of spin-orbit coupling, is still
lacking. Here we show that a class of novel topological semimetals with
point/line nodes can emerge in the presence of an off-centered rotation/mirror
symmetry whose symmetry line/plane is displaced from the center of other
symmorphic symmetries in nonsymmorphic crystals. Due to the partial translation
perpendicular to the rotation axis/mirror plane, an off-centered
rotation/mirror symmetry always forces two energy bands to stick together and
form a doublet pair in the relevant invariant line/plane in momentum space.
Such a doublet pair provides a basic building block for emerging topological
semimetals with point/line nodes in systems with strong spin-orbit coupling.
