Buckyballs directly probe inter-valley coherence in moire heterostructures

Abstract

Moire materials like twisted bilayer graphene have emerged as a rich playground of strongly correlated physics, where the effect of interactions can be drastically enhanced by tuning the non-interacting density of states via twist angle. A key feature of bilayer graphene at small twist angles is that the two valleys of graphene are effectively decoupled at the non-interacting level, leading to an emergent U(1) symmetry corresponding to the conservation of valley-charge. Theoretical studies invariably find that short-ranged interactions tend to break this symmetry, leading to inter-valley coherence (IVC). Here, I propose an experimental signature of IVC order by coupling with a closely related system where valley-mixing is guaranteed - the pentagonal disclinations of graphene. Such disclinations are naturally found in fullerenes. The tunneling current between twisted bilayer graphene and an appropriately oriented large buckyball has a sharp signature of the onset of IVC order. Successfully integrating these fullerenes in a scanning tunneling microscope would allow spatial resolution of the IVC order parameter on a sub-moire scale. Experimentally constraining the broken symmetries and order parameters in the multitude of emerging correlated insulators is a crucial first step to understanding the superconductivity that develops on doping these exotic `normal' states.