Half- and quarter-metals in rhombohedral trilayer graphene

Haoxin Zhou, Tian Xie, Areg Ghazaryan, Tobias Holder, James R. Ehrets, Eric M. Spanton, Takashi Taniguchi, Kenji Watanabe, Erez Berg, Maksym Serbyn and Andrea F. Young ()
Additional contact information
Haoxin Zhou: University of California
Tian Xie: University of California
Areg Ghazaryan: Institute of Science and Technology
Tobias Holder: Weizmann Institute of Science
James R. Ehrets: University of California
Eric M. Spanton: University of California
Takashi Taniguchi: National Institute for Materials Science
Kenji Watanabe: National Institute for Materials Science
Erez Berg: Weizmann Institute of Science
Maksym Serbyn: Institute of Science and Technology
Andrea F. Young: University of California

Nature, 2021, vol. 598, issue 7881, 429-433

Abstract: Abstract 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 systems1–5. Here we show that gate-tuned van Hove singularities in rhombohedral trilayer graphene6 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 model7 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 substrate5,8. 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 materials4,5,9,10 to be fundamentally itinerant in nature.

Date: 2021
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DOI: 10.1038/s41586-021-03938-w

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