Effect of disorder on surface states of Weyl nodal loop semimetals

We address the effect of disorder on the topological surface (“drumhead”) states of a Weyl nodal loop semimetal. Two types of disorder are considered: 

Anderson disorder breaks chiral symmetry, so, a topological index is not defined. Weak  disorder produces the algebraic decay of the E=0 surface states into the bulk, and a broadening of the low energy density of surface states by degeneracy lifting. This behavior persists up to the bulk semimetal/metal transition at the critical disorder, Wc , above which all  surface states hybridize with bulk states and become extended.

Weak  chiral disorder induces a diffusive metal at zero energy and, surprisingly, enhances the number of topological surface states as confirmed by the winding number and the low energy DOS of the open system. The topological surface states are exponentially localized into the bulk, and exhibit a multifractal structure on the surface.

This is a novel topological metal hosting topological multifractal surface states. The number of surface states decreases either algebraically with disorder, or  exponentially if  the latter only affects intracell hoppings.