Continuous correlated states and dual-flatness in a moir\'e heterostructure

TL;DR

This paper explores how local and global flat bands in twisted monolayer-bilayer graphene lead to correlated metallic states with symmetry breaking, revealing new insights into moiré physics beyond traditional integer fillings.

Contribution

It demonstrates the coexistence of global flat bands and local band flattening in twisted graphene, enabling correlated phenomena at non-integer fillings without a global gap.

Findings

Observation of symmetry breaking and valley polarization at non-integer fillings.

Detection of anomalous Hall responses indicating time-reversal symmetry breaking.

Identification of dual-flatness as a principle extending moiré physics.

Abstract

Many-body effects in condensed matter yield novel quantum states when the electronic density of states is enhanced. A vivid example is flat bands, which suppress kinetic energy and let interactions dominate, when they are filled with an integer number of electrons in moire systems. Yet flat bands and commensurate fillings are not the only conditions for correlated phenomena. Situations may occur where the band structure develops locally enhanced density of states, leading to strong correlations even at non-integer fillings, although such cases often yield pseudogaps that make detection elusive. Here we demonstrate that small-angle twisted monolayer-bilayer graphene combines moire-induced global flat band and additional local band flattening. Their coexistence allows direct comparison of correlated effects. The global route stabilizes commensurate states, while the local mechanism produces nearly flat bands, lifting degeneracy and generating symmetry breaking at non-integer fillings, yet without opening a global gap. Because there is no global gapped signature, the system remains metallic, but the effect reveals itself in anomalous Hall responses, signaling time-reversal symmetry breaking and valley polarization. Our results demonstrate dual-flatness as a guiding principle, extending moire physics beyond commensurate fillings and identifying topological transport as a probe of gapless correlated metals.