Intrinsic and Extrinsic Photogalvanic Effects in Twisted Bilayer Graphene

Area of Science:

• Condensed Matter Physics
• Materials Science
• Quantum Optics

Background:

• Twisted bilayer graphene (TBG) possesses a chiral lattice structure with D6 symmetry.
• Intrinsic photogalvanic effects in TBG are observed only at off-normal incidence.
• Extrinsic photogalvanic effects can be induced by substrate interactions or gate potentials.

Purpose of the Study:

• To compute and analyze intrinsic photogalvanic effects in TBG.
• To investigate extrinsic photogalvanic effects and their relation to substrate coupling and electric fields.
• To explore the constraints imposed by particle-hole symmetry on photocurrents.

Main Methods:

• Theoretical computation of intrinsic photogalvanic effects.
• Analysis of extrinsic effects induced by substrate and gate potentials.
• Investigation of particle-hole symmetry implications on photocurrents.

Main Results:

• Intrinsic photogalvanic effects were computed and shown to reverse sign at the magic angle, indicating band inversion at the Γ point.
• Extrinsic effects were analyzed, demonstrating their utility in quantifying substrate coupling and electric displacement field strengths.
• Approximate particle-hole symmetry was shown to impose strict constraints on the chemical potential dependence of all photocurrents.

Conclusions:

• A detailed comparison of intrinsic and extrinsic photocurrents provides significant information about the band structure of TBG.
• This comparison serves as a benchmark for constraining symmetry-breaking patterns in correlated states of TBG.