Higher-Order Topology in the Superconducting State of Nodal-Line Spin-Gapless Semimetals

We study the topological properties of a nodal-line spin gapless semimetal superconductor. In the normal state, the low energy states are completely spin polarized so that the p + i p -wave pairing symmetry is considered. In the superconducting state, the energy bands are fully gapped in both the system bulk and at the system surface. However, when two open boundaries are considered, the gapless states emerge at the system hinges, indicating that the system is actually a higher-order topological superconductor.