Angle-tuned Gross-Neveu quantum criticality in twisted bilayer graphene

Area of Science:

• Condensed Matter Physics
• Quantum Materials
• Materials Science

Background:

• Quantum many-body states in twisted bilayer graphene at the magic angle are well-understood.
• The phase diagram and excitations concerning twist angle and permittivity remain largely unknown.

Purpose of the Study:

• Investigate the quantum phase diagram and excitations in twisted bilayer graphene as a function of twist angle and permittivity.
• Characterize the quantum phase transition in charge-neutral twisted bilayer graphene.

Main Methods:

• Utilized a state-of-the-art momentum-space continuous-field quantum Monte Carlo method.
• Incorporated long-ranged Coulomb interactions and flat bands' quantum metrics.
• Simulated system sizes up to 15 × 15.

Main Results:

• Identified an angle-tuned quantum phase transition from a Kramers intervalley-coherent insulator to a Dirac semimetal at critical angles around 1.2°.
• Observed the evolution of single-particle spectra from a gapped state at Γ to touching points at Brillouin zone corners with increasing angle.
• Determined the transition belongs to fermionic Gross-Neveu criticality based on free energy and order parameters.

Conclusions:

• The quantum phase transition in twisted bilayer graphene is angle-tuned and robust against variations in permittivity and interlayer hopping.
• The transition is characterized by fermionic Gross-Neveu criticality.
• Provides a comprehensive understanding of the phase diagram and excitations in twisted bilayer graphene beyond the magic angle.