Two-atom-thin topological crystalline insulators lacking out of plane inversion symmetry

Area of Science:

• Condensed Matter Physics
• Materials Science
• Quantum Mechanics

Background:

• Topological crystalline insulators (TCIs) are materials with unique electronic properties.
• Topological protection of surface states is often linked to crystal symmetries.

Purpose of the Study:

• To demonstrate a two-dimensional topological crystalline insulator (TCI) with single unit cell thickness.
• To investigate the role of in-plane rotational symmetry in protecting topological surface states.

Main Methods:

• Theoretical modeling of a TCI system.
• Analysis of bulk and surface electronic band structures.
• Topological characterization using Pfaffians.

Main Results:

• Zero-energy metallic surface states were observed in a single unit cell thick TCI.
• These surface states persist even after removing all in-plane rotational symmetries.
• Topologically protected two-fold energy degeneracies in the bulk Hamiltonian remain at the single unit cell limit.
• The topological properties of the bulk and single unit cell thick phases are invariant.

Conclusions:

• Tetragonal symmetry is not essential for creating zero-energy metallic surface states in TCIs.
• A robust topological phase can exist in a two-atom-thick TCI, bridging bulk and 2D topological properties.