Double Dirac nodal lines enforced by multiple nonsymmorphic symmetries

Abstract

The topological quantum states of matter can be characterized by the geometric form and number of symmetry-enforced band degeneracy, such as nodal points, lines, and surfaces from twofold up to eightfold degeneracy. Here, we report the observation of double Dirac (fourfold) nodal lines stabilized by multiple nonsymmorphic symmetries in puckered honeycomb systems, black phosphorus, and metal monochalcogenides. By angle-resolved photoemission spectroscopy, we found a fourfold (Dirac) nodal line running along the armchair zone boundary of black phosphorus, reproduced by first-principles band calculations and proven by our symmetry analysis to be protected by space-time inversion and glide-mirror symmetries. The presence of the multiple glide-mirror symmetries in its binary counterpart, GeS, diversifies into five Dirac nodal lines, two of which come closer at the zigzag zone boundary to form a nearly eightfold nodal line. Our results demonstrate the correspondence between nonsymmorphicity and band degeneracy with puckered honeycomb systems. © 2024 American Physical Society.