Plethora of Coexisting Topological Band Degeneracies in Nonsymmorphic Molecular Crystal OsOF$_5$

Abstract: In band theory of solids, degeneracies manifest themselves as point, line, and surface objects. The topological nature of band degeneracies has recently been appreciated since the recognition of gapless topological quantum phases of matter. Based on first-principles calculations, we report the topological semimetal phase in osmium oxyhalide OsOF$_5$ that exhibits zero-dimensional Weyl points, one-dimensional nodal lines, and two-dimensional nodal surfaces. The semimetal phase is ensured by the electron filling constraint while band degeneracies of various dimensionalities are protected by the nonsymmorphic crystalline symmetries. Focusing on two nodal loops crossing the Fermi level, we demonstrate their topological protection by $\pi$ Berry phase and describe the drumhead surface states topologically connected by nontrivial $\mathbb{Z}_2$ index. Our prediction highlights OsOF$_5$ as a unique topological material in which a rich variety of topological degeneracies exists in a narrow energy range close to the Fermi level owing to the molecular crystal structure of this material.