Universality and stability of the edge states of chiral-symmetric topological semimetals and surface states of the Luttinger semimetal

We theoretically demonstrate that the chiral structure of the nodes of nodal semimetals is responsible for the existence and universal local properties of the edge states in the vicinity of the nodes. We perform a general analysis of the edge states for an isolated node of a 2D semimetal, protected by {em chiral symmetry} and characterized by the topological winding number $N$. We derive the asymptotic chiral-symmetric boundary conditions and find that there are $N+1$ universal classes of them. The class determines the numbers of flat-band edge states on either side off the node in the 1D spectrum and the winding number $N$ gives the {em total} number of edge states. We then show that the edge states of chiral nodal semimetals are {em robust}: they persist in a finite-size {em stability region} of parameters of chiral-asymmetric terms. This significantly extends the notion of 2D and 3D topological nodal semimetals. We demonstrate that the Luttinger model with a quadratic node for $j=frac32$ electrons is a 3D topological semimetal in this new sense and predict that $alpha$-Sn, HgTe, possibly Pr$_2$Ir$_2$O$_7$, and many other semimetals described by it are topological and exhibit surface states.