Nodal line fermions in magnetic oxides

Using symmetry considerations, we show that nodal rings in a certain class of magnetic oxides are guaranteed to remain gapless in the presence of spin-orbital coupling (SOC). The condition is that the compound must possess a mirror plane perpendicular to its intrinsic magnetic moment. We also show by first-principles calculations that a number of widely used compounds with a cubic phase, such as Fe3O4, CrO2, Li2VO4, Li2TiO4, and Cu2TiO4, belonging to the symmorphic magnetic group D4h(C4h), possess nodal rings with this guaranteed feature. Furthermore, we investigate drumhead surface states of these compounds bounded by a semi-infinite (001) surface. The drumhead surface states in some of our proposed materials (e.g., CrO2), located near the Fermi level, are easily detectable and guaranteed to be intact in the presence of SOC. The findings are important to applications of topological materials to quantum transport. This work also demonstrates the use of symmetry considerations as a powerful pathway for discovering stable node-line fermions and nontrivial surface states in the presence of SOC.