Electrically controllable valence-conduction band reversals in helical trilayer graphene

TL;DR

This paper reveals how electron interactions in helical trilayer graphene cause reversible switching of valence and conduction bands, leading to new magnetic and electronic phase transitions within metallic regimes.

Contribution

It uncovers a novel interaction-driven mechanism causing cyclic valence-conduction band reversals in HTG, supported by experimental magnetometry and theoretical calculations.

Findings

Detection of sharp magnetic signatures indicating band reversals

Observation of magnetic hysteresis linked to band transitions

Reorganization of flat bands driven by electron interactions

Abstract

In moir\'e graphene systems, electronic interactions lift spin and valley degeneracies, leading to symmetry-broken ground states. In helical trilayer graphene (HTG), we uncover a distinct interaction-driven mechanism in which the roles of sublattice-polarized valence and conduction bands are cyclically reversed. Using scanning nano-SQUID magnetometry, we detect a series of sharp magnetic signatures consistent with seesaw-like transitions, where occupied and unoccupied valence and conduction bands interchange repeatedly with doping, accompanied by a novel form of magnetic hysteresis. These transitions occur entirely within metallic regimes and leave only weak fingerprints in transport measurements. Self-consistent Hartree-Fock calculations reveal that interactions reorganize all eight low-energy flat bands, driving abrupt changes in orbital magnetization. Our results establish HTG as the first system where electronic interactions provide doping-controlled access to all three internal degrees of freedom - spin, valley, and sublattice - introducing a new class of correlated phase transitions.