José Pizarro: Internal screening and dielectric engineering in magic-angle twisted bilayer graphene

Ort : NW1 S1360
Beginn : 30. April 2019, 15:00 Uhr
Ende : 30. April 2019, 16:00 Uhr

J. M. Pizarro
Institute for Theoretical Physics & Bremen Center for Computational Material
Sciences, University of Bremen, Germany

Internal screening and dielectric engineering in
magic-angle twisted bilayer graphene

Stacking two graphene layers at a twist angle θ on top of each other leads to twisted bilayer
graphene (tBLG) featuring a moiré pattern with an intricate emergent low energy electronic
structure. For small twist angles θ<2⁰ the resulting superlattices host several thousand atoms
per unit cell. In this situation, the electronic bands around the charge neutrality point (CNP)
become very flat, which facilitates strong correlation effects. Recent experiments reported the
emergence of possibly unconventional superconducting and insulating states in magic-angle
tBLG (MA-tBLG) at different levels of doping. The insulating states occur for commensurate
fillings at both electron and hole dopings, signaling a possible Mott-Hubbard origin. Around
these insulating states, superconductivity emerges, resembling the phase diagram of high-Tc
cuprate superconductors and other unconventional superconductors.
While the impact of external parameters such as doping or magnetic field can be conveniently
modified and analyzed, the all-surface nature of the quasi-2D electron gas combined with its
intricate internal properties pose a challenging task to characterize the quintessential nature
of the different insulating and superconducting states found in experiments. We analyze the
interplay of internal screening and dielectric environment on the intrinsic electronic
interaction profile of MA-tBLG. We find that interlayer coupling generically enhances the
internal screening. The influence of the dielectric environment on the effective interaction
strength depends decisively on the electronic state of MA-tBLG. Thus, we propose the
experimental tailoring of the dielectric environment, e.g. by varying the capping layer
composition and thickness, as a promising pursuit to provide further evidence for resolving the
hidden nature of the quantum many-body states in MA-tBLG.