Spin-orbit-free topological insulators without time-reversal symmetry

Area of Science:

• Condensed Matter Physics
• Materials Science
• Crystallography

Background:

• Topological insulators (TIs) typically rely on spin-orbit coupling (SOC) for their unique properties.
• Understanding crystalline symmetries is crucial for classifying topological phases of matter.
• Robust surface modes are a hallmark of topological materials.

Purpose of the Study:

• To identify novel 3D topological insulators protected by crystalline symmetries.
• To explore topological phases in systems without spin-orbit coupling.
• To introduce new topological invariants for characterizing these phases.

Main Methods:

• Systematic exploration of the 32 crystallographic point groups.
• Identification of topological invariants like halved mirror chirality and bent Chern number.
• Analysis of surface modes protected by point group symmetries.

Main Results:

• Discovery of the first 3D topological insulators without spin-orbit coupling for C_{nv} groups (n=3, 4, 6).
• Surface modes are protected by purely crystalline symmetries, not time-reversal symmetry.
• A Weyl semimetallic phase acts as an intermediate between distinct gapped phases.

Conclusions:

• Crystalline symmetries alone can protect topological surface states in 3D materials.
• New topological invariants (halved mirror chirality, bent Chern number) are introduced.
• Findings are applicable to electronic, photonic, and ultracold atom systems.