We demonstrate a rare nonsymmorphic crystalline symmetry-protected nodal-line semimetallic phase in ${\text{SrIrO}}_3$ by acquiring a spatial dependence of the terahertz spectral structure. An extremely low momentum-relaxation scattering rate of $\sim 8.32 {\text{cm}}^{-1}$ emerging from high-mobility Dirac electrons, diversity in the carrier dynamics controlled by the orientation and symmetry of epitaxial thin films, and a group theory analysis corroborate the formation of this phase. The presence of Dirac electrons only along selected crystal orientations of this three-dimensional semimetal uniquely interrelates the symmetry-enforced band crossings, a template that can explore the prevalence of this superior topological phase in complex systems.

• Received 21 November 2021
• Revised 12 September 2022
• Accepted 1 December 2022

DOI:https://doi.org/10.1103/PhysRevB.106.245135