  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.
