Studying interplay between non-linear electron transport and correlated physics in graphene-based flat band systems

Recent observations of correlated physics such as superconductivity, orbital ferromagnetism, and many-body insulating states in twisted graphene devices drove enormous enthusiasm in the study of flat band systems. It is commonly believed that such physics can be observed only under extremely small excitation currents, thus so far flat band systems are studied mostly in a linear response regime. Here we make the first step towards the understanding of non-linear properties of those systems and studying electron transport under high bias in small-angle twisted monolayer-bilayer graphene (tMBG) devices [1,2].

 First, we observe a non-linear current propagation around the single body gaps (around fillings ν= -4, 0, 4 per Moiré unit cell). We show that this non-linear regime is governed by the out-of-equilibrium criticalities general to graphene superlattices [3]. Second, when we tune the Fermi level close to the many-body correlated insulating states (ν=1,2, and 3 electrons per Moiré unit cell) we observe similar out-of-equilibrium criticalities with sharp peaks in differential resistance and superconducting-like I-V curves. These criticalities originate from the strong shift of the Fermi surface at the many-body band edge.

Overall, we show that many-body gaps in tMBG are robust against the significant shift of the Fermi surface happening under high current. Our results provide the first insights into the interplay between high bias electron transport and correlated physics in flat band systems. 

[1] Nat. Phys. 17, 374-380 (2021)

[2] Nat. Phys. 17, 619-626 (2021) 

[3] arXiv:2106.12609