Surface States of Perovskite Iridates AIrO$_3$; Signatures of Topological Crystalline Metal with Nontrivial $\mathbb{Z}_2$ Index

TL;DR

This study uses first-principles calculations to reveal orthorhombic perovskite iridates host both lattice-symmetry-protected flat surface states and time-reversal symmetry-protected Dirac cones, indicating complex topological phases.

Contribution

It demonstrates the coexistence of lattice- and time-reversal symmetry-protected topological surface states in orthorhombic iridates, a novel finding in topological materials.

Findings

Identification of flat surface states on side surfaces protected by lattice symmetries

Discovery of Dirac cone on top surface indicating time-reversal symmetry protection

Confirmation of nontrivial $ ext{Z}_2$ index indicating topological insulator characteristics

Abstract

There have been increasing efforts in realizing topological metallic phases with nontrivial surface states. It was suggested that orthorhombic perovskite iridates are classified as a topological crystalline metal (TCM) with flat surface states protected by lattice symmetries. Here we perform first-principles electronic structure calculations for epitaxially stabilized orthorhombic perovskite iridates. Remarkably, two different types of topological surface states are found depending on surface directions. On side surfaces, flat surface states protected by lattice symmetries emerge, manifesting the topological crystalline character. On the top surface, on the other hand, an unexpected Dirac cone appears, indicating surface states protected by a time-reversal symmetry, which is confirmed by the presence of a nontrivial topological $\mathbb{Z}_2$ index. These results suggest that the orthorhombic iridates are unique systems exhibiting both lattice- and global-symmetry-protected topological phases and surface states. Transitions to weak and strong topological insulators and implications of surface states in light of angle resolved photoemission spectroscopy are also discussed.