Fragile topology in nodal-line semimetal superconductors

Abstract

We study the topological properties of the nodal-line semimetal superconductor. The single band inversion and the double band inversion coexist in an s-wave nodal-line semimetal superconductor. In the single/double band inversion region, the system is in a stable/fragile topological state. The two topological invariants describing these two topological states are coupled to each other, leading to the coupled edge states. The stable topological state is indexed by Z(d=1), while the fragile topological state is characterized to be Z ⊗ Z(d = 1). In addition, the s-wave nodal-line semimetal superconductor has a nontrivial Z4 = 2 topological invariant, indicating that it is a inversion symmetry protected second order topological crystalline superconductor. While the p-wave nodal-line semimetal belongs to a pure fragile topological superconductor due to the double band inversion. The vortex bound states and the surface impurity effects are studied and they can be used to distinguish the different pairing states and identify the fragile topology of the system. Remarkably, we propose that vortex line in the nodal-line semimetal superconductor is a one dimensional fragile topological state protected by the spatial symmetry.