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Half- and quarter-metals in rhombohedral trilayer graphene
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Journal Article
Half- and quarter-metals in rhombohedral trilayer graphene
2021
Overview
Ferromagnetism is most common in transition metal compounds where electrons occupy highly localized
d
orbitals. However, ferromagnetic order may also arise in low-density two-dimensional electron systems
1
–
5
. Here we show that gate-tuned van Hove singularities in rhombohedral trilayer graphene
6
drive spontaneous ferromagnetic polarization of the electron system into one or more spin and valley flavours. Using capacitance and transport measurements, we observe a cascade of transitions tuned to the density and electronic displacement field between phases in which quantum oscillations have fourfold, twofold or onefold degeneracy, associated with a spin- and valley-degenerate normal metal, spin-polarized ‘half-metal’, and spin- and valley-polarized ‘quarter-metal’, respectively. For electron doping, the salient features of the data are well captured by a phenomenological Stoner model
7
that includes valley-anisotropic interactions. For hole filling, we observe a richer phase diagram featuring a delicate interplay of broken symmetries and transitions in the Fermi surface topology. Finally, we introduce a moiré superlattice using a rotationally aligned hexagonal boron nitride substrate
5
,
8
. Remarkably, we find that the isospin order is only weakly perturbed, with the moiré potential catalysing the formation of topologically nontrivial gapped states whenever itinerant half- or quarter-metal states occur at half- or quarter-superlattice band filling. Our results show that rhombohedral graphene is an ideal platform for well-controlled tests of many-body theory, and reveal magnetism in moiré materials
4
,
5
,
9
,
10
to be fundamentally itinerant in nature.
A study shows that rhombohedral graphene is an ideal platform for well-controlled tests of many-body theory and reveals that magnetism in moiré materials is fundamentally itinerant in nature.
