  Magnetic topological materials and their physical signatures are a focus of
current research. Here, by first-principles calculations and symmetry analysis,
we reveal topological semimetal states in an existing antiferromagnet ThMn2Si2.
Depending on the N\'eel vector orientation, the topological band crossings near
the Fermi level form either a double-nodal loop or two pairs of Dirac
points,which are all fourfold degenerate and robust under spin-orbit coupling.
These topological features produce large Berry connection polarizability, which
leads to enhanced nonlinear transport effects. Particularly, we evaluate the
third order current response, which dominates the transverse charge current. We
show that the nonlinear response can be much more sensitive to topological
phase transitions than linear response, which offers a powerful tool for
characterizing magnetic topological semimetals.
