Exceptional Magic Angles in Non-Hermitian Twisted Bilayer Graphene

Area of Science:

• Condensed Matter Physics
• Quantum Materials

Background:

• Twisted bilayer graphene (TBG) exhibits correlated and topological phases due to flat bands near the magic angle.
• The impact of non-Hermiticity (dissipation/environment coupling) on TBG's electronic properties remains largely uninvestigated.

Purpose of the Study:

• To introduce and analyze a non-Hermitian (NH) model for twisted bilayer graphene.
• To explore the emergence of novel electronic phases and magic angles in dissipative TBG systems.

Main Methods:

• Developed a simplified NH-TBG model by incorporating hopping imbalance in graphene monolayers.
• Generalized the Bistritzer-MacDonald approach for NH band structure calculations.

Main Results:

• Discovered "exceptional magic angles" where band structures transition to purely imaginary energies, yielding flat bands with infinite lifetimes.
• Identified a Hermitian magic angle where energy's imaginary part is maximized, aligning with traditional TBG.
• Observed band flattening persists between these exceptional angles.

Conclusions:

• Flat bands in TBG are robust even in the presence of dissipation.
• Suggests potential for experimental verification using optical lattices with gain and loss.
• Opens avenues for studying dissipative effects, electronic topology, and interactions in NH moiré bands.