Experimental evidence for orbital magnetic moments generated by moir\'e-scale current loops in twisted bilayer graphene

TL;DR

This study provides experimental evidence of orbital magnetic moments in twisted bilayer graphene caused by moire-scale current loops, revealing a new form of orbital magnetism linked to electron interactions and valley degrees of freedom.

Contribution

First direct measurement of orbital magnetic moments in twisted bilayer graphene, demonstrating their origin from moire-scale current loops and their response to magnetic fields.

Findings

Orbital magnetic moment of about 10.7 μB per moire supercell.

Valley degeneracy is lifted by electron-electron interactions near the VHS.

Large linear response of valley splitting to magnetic fields.

Abstract

A remarkable property of twisted bilayer graphene (TBG) with small twist angle is the presence of a well-defined and conserved low-energy valley degrees of freedom1, which can potentially bring about new types of valley-associated spontaneous-symmetry breaking phases. Electron-electron (e-e) interactions in the TBG near the magic angle 1.1 degree can lift the valley degeneracy, allowing for the realization of orbital magnetism and topological phases2-11. However, direct measurement of the orbital-based magnetism in the TBG is still lacking up to now. Here we report evidence for orbital magnetic moment generated by the moire-scale current loops in a TBG with a twist angle {\theta} ~ 1.68 degree. The valley degeneracy of the 1.68 degree TBG is removed by e-e interactions when its low-energy van Hove singularity (VHS) is nearly half filled. A large and linear response of the valley splitting to magnetic fields is observed, attributing to coupling to the large orbital magnetic moment induced by chiral current loops circulating in the moire pattern. According to our experiment, the orbital magnetic moment is about 10.7 uB per moire supercell. Our result paves the way to explore magnetism that is purely orbital in slightly twisted graphene system.