Seesaw switching of topological bands in helical trilayer graphene

The richness of the electronic phenomena in moiré quantum matter stems from their unique combination of flat band structure, strong interactions, and topology. Yet, the microscopic determination of these ingredients remains highly challenging. The recent advent of multi-moiré systems elevates this challenge even further, since the large supermoiré length scales result in strong spatial modulation of the local properties that cannot be adequately studied by global methods. Scanning SQUID-on-tip microscopy has been recently demonstrated to provide direct access to the spatially resolved band structure [1,2], electronic correlations [3], and local band topology [4] in moiré materials.

We will present the first nanoscale study of the band and topology evolution in a multi-moiré helically twisted trilayer graphene (HTG) system. This system is expected to host two non-equivalent topological flat bands: a valence flat band that is highly polarized to the B sublattice, with C=±2, and a conduction flat band that is highly polarized to the A sublattice, with C=±1. By imaging the local magnetization, we reveal an unusually rich magnetic phase diagram that is invisible to transport measurements. We find multiple crossovers and sharp transitions as a function of the displacement field and filling factor, which mark repeated seesaw-like switching between the topologically distinct valence and conduction bands induced by a sequence of Stoner instabilities. Further, we discover a novel hysteresis mechanism, where instead of being isolated to the vicinity of integer fillings, the hysteresis is bounded strictly by consecutive Stoner phase transitions. Moreover, the phase transitions are found to be position-dependent, emphasizing the role of local lattice relaxation in the supermoiré structure. These findings establish the multi-moiré systems as a prolific platform for discovering new strongly correlated phenomena in quantum materials.