Are Weyl and Dirac points topological defects in nodal semimetals? Recently, I heard  the Weyl and Dirac points are topological defects in nodal semimetals. I do not really get it. And the definition of topological defects is confusing to me.
Are the topological defects equal to gapless boundary states?
Are Weyl and Dirac points topological defects in nodal semimetals?
How about the nodal line in the topological semimetal? Is it also topological defect?
 A: now topological defect is one thing and topological quasi particle excitation is another thing.
topological defect is introduced extrinsically and can not be removed smoothly it is not an excitation. One example is, vortices in bose super fluis they are topological defects  also they feel each other, they have a pseudo interaction. they are introduced extrinsically.
however, vortices in superconductors are not topological defects, they do not feel each other, they are just excitations to the system.  Some of the solutions of hamiltonian supports these vortices but there are also solutions without these vortices as well. They are just quasi particle excitations.
so in the weyl semi metal case, those band touchings are also topological defects, they are introduced extrinsically, but it is interesting because these topological defects are in momentum space, and in momentum space they have non trivial topological charge as you expect from a topological defect. So, those band touchings are indeed topological defects in the momentum space. 
however, topological defect, and topological quasi particle excitation are different.
