  Nodal-line semimetals are characterized by a kind of topologically nontrivial
bulk-band crossing, giving rise to almost flat surface states. Yet, a direct
evidence of the surface states is still lacking. Here we study theoretically
impurity effects in topological nodal-line semimetals based on the T-matrix
method. It is found that for a bulk impurity, some in-gap states may be induced
near the impurity site, while the visible resonant impurity state can only
exist for certain strength of the impurity potentials. For a surface impurity,
robust resonant impurity states exist in a wide range of impurity potentials.
Such robust resonant states stem from the topological protected weak dispersive
surface states, which can be probed by scanning tunneling microscopy, providing
a strong signature of the topological surface states in the nodal-line
semimetals.
