  Based on symmetry analysis, we propose that filling-enforced Dirac loops
(FEDLs) in non-magnetic systems exist and only exist in only five space groups
(SGs), namely, SG.57, SG.60, SG.61, SG.62 and SG.205. %, respectively.
  We explore all possible configurations of the FEDLs in these space groups,
and classify them accordingly. Furthermore, we study the evolutions of the
FEDLs under various types of symmetry-breaking perturbations, such as an
applied strain or an external field. The results show that FEDL materials can
serve as parent materials of both topological semimetals hosting nodal
points/loops, and topological insulators/topological crystalline insulators. By
means of first-principles calculations, many materials possessing FEDLs are
predicted.
