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Condensed Matter Seminar: Superconductivity, Topology and Correlated Insulators in Magic-Angle Twisted Bilayer Graphene–How Much Magic is Needed?
Speaker Prof. Stevan Nadj-Perge, California Institute of Technology
Date 1 April 2021 (Thursday)
Time 11:00 - 12:30
Venue Online (Zoom)

Twisted bilayer graphene (TBG) with a rotational misalignment angle close to the magic angle of 1.1 degrees features isolated flat electronic bands that host various correlated insulating, superconducting, ferromagnetic, and topological phases. However, the origin of these phases and the relation between them remain elusive because of their sensitivity to microscopic details in TBG, such as strain, twist angle disorder, and dielectric environment.

In this talk, I will discuss our transport and scanning tunneling microscopy (STM) experiments that explore the stability of various correlated phases with respect to twist angle and its deviation from the magic-angle value. First, we will show that the addition of tungsten diselenide (WSe2) monolayer between TBG and its dielectric environment stabilizes superconductivity down to twist angles as small as 0.79 degrees. Importantly, for angles in this range, both the correlated insulating states and the band gaps between flat and dispersive bands disappear, leading to metallic behavior across the accessible range of electron density. These findings significantly constrain the theoretical explanations for the emergence of superconductivity in TBG and its relation to other correlated phases. In the second part of the talk, we will discuss STM measurements that reveal how the interplay between strong interactions and non-uniform charge distribution in TBG leads to the flattening of the TBG bands. Close to the magic angle, this interaction-driven band flattening gives rise to robust correlated insulating and topological Chern phases that emerge in zero and finite magnetic fields. Our results highlight the role of band structure renormalization, alongside spontaneous symmetry breaking, in the formation of strongly correlated and topological TBG phases.

Meeting Link: To request for meeting link, please write to phweb@ust.hk.