Less is More: Vacancy-Triggered Robust Topological Nodal-Line Semimetals

A nodal-line semimetal (NLSM) is suppressed by perturbations unless it is enforced by a nonsymmorphic symmetry. We show that two-dimensional materials can realize robust NLSMs when vacancies are periodically distributed through the lattice. We propose an effective 2D model and a symmetry analysis to demonstrate that the resulting NLSM-phase is topological and enforced by a nonsymmorphic glide plane. We test our prediction by carrying out a DFT investigation of borophene, a boron honeycomb-like sheet. While the Dirac cones of pristine borophene are shown to be gapped out by spin-orbit coupling and by magnetic exchange, robust nodal lines emerge in the spectrum when selected atoms are periodically removed along a certain direction on the lattice. Our findings offer a paradigm shift to the design of NLSMs: Instead of searching for complicated nonsymmorphic materials, robust NLSMs may be realized simply by removing atoms from ordinary symmorphic crystals.