Peculiar symmetry-protected electronic dispersions in two-dimensional materials

Area of Science:

• Condensed Matter Physics
• Materials Science
• Solid-State Physics

Background:

• Symmetry in materials can lead to exotic low-energy electronic spectra beyond known Dirac or quadratic dispersions.
• Unusual quasiparticles can emerge from specific band degeneracies and symmetries in crystalline materials.

Purpose of the Study:

• To systematically study effective Hamiltonians and dispersions at high-symmetry points with fourfold band degeneracy.
• To identify and characterize new types of quasiparticles arising from specific symmetry conditions in two-dimensional (2D) crystals.
• To explore the potential realization of these novel fermions in existing and exfoliated layered materials.

Main Methods:

• Systematic theoretical analysis of low-energy effective Hamiltonians and band dispersions.
• Identification of symmetry groups (noncentrosymmetric layer groups) admitting novel dispersions.
• Density functional theory (DFT) based calculations to confirm predictions in specific materials.
• Analytical calculation of the density of states (DOS) for the predicted quasiparticles.

Main Results:

• Discovery of a new massless fermion dispersion, named poppy flower (PF) fermion, in 2D non-magnetic crystals with spin-orbit coupling (SOC) belonging to specific noncentrosymmetric layer groups.
• Identification of conditions for hosting fortune teller (FT)-like fermions in 2D crystals with SOC, beyond systems without SOC.
• Experimental confirmation of FT fermions in exfoliated BiIO₄ monolayer, which exhibits the required symmetry (pb2₁a).
• PF fermions exhibit a semimetallic density of states, while FT fermions show metallic characteristics with non-zero DOS at band contact energy.

Conclusions:

• The study reveals that material symmetry can host rich low-energy spectra, including the novel PF fermion.
• Layered materials with specific noncentrosymmetric symmetries are promising candidates for realizing PF and FT fermions.
• The findings provide a pathway for discovering new electronic properties and designing materials with tailored characteristics.