Near-Field Microscopy in Moiré Graphene Heterostructures

Speaker: Petr Stepanov
Affiliation: Notre Dame
Date: 10/23/25

Abstract: 
The discovery of superconductivity in magic-angle twisted bilayer graphene (MATBG) has reshaped our understanding of correlated quantum materials. When two graphene sheets are stacked and twisted by just the right “magic” angle, their electronic bands flatten, giving rise to a playground of exotic quantum phenomena - including superconductivity, correlated insulating states, magnetic ordering, and nontrivial topological phases. Unlike traditional strongly correlated systems, these two-dimensional heterostructures offer remarkable experimental tunability: carrier density, dielectric screening, and proximity to metallic gates can all be precisely controlled, enabling systematic exploration of emergent electronic phases. In the first part of this talk, I will discuss ongoing puzzles in MATBG research - in particular, the discrepancies between transport and scanning tunneling microscopy measurements that challenge our understanding of the spatial localization and orbital nature of the flat-band states. To address these challenges, we have developed a new experimental approach based on local thermoelectric microscopy. Using a cryogenic near-field optical microscope with an infrared-illuminated oscillating AFM tip, we create a nanoscale thermal hot spot on the sample surface and measure the resulting local photovoltage. This allows us to spatially map variations in the Seebeck coefficient across the sample. Our results reveal a striking asymmetry in quasiparticle lifetimes across correlated minigaps - a possible signature of momentum-space localization effects in these flat-band systems. In the second part, I will present our recent microscopy studies of high-angle (~5.5°) twisted bilayer graphene (HATBG). Here, we directly visualize hybrid plasmon-phononpolaritons and use them to extract twist-angle–dependent group velocities in the electronic bands. These findings demonstrate a clear departure of HATBG from the conventional “monolayer-plus-monolayer” picture, shedding new light on the collective excitations and electronic structure in the regime beyond the magic angle.