Fluctuating magnetism and Pomeranchuk effect in multilayer graphene

Ludwig Holleis, Tian Xie, Siyuan Xu, Haoxin Zhou, Caitlin L. Patterson, Archisman Panigrahi, Takashi Taniguchi, Kenji Watanabe, Leonid S. Levitov, Chenhao Jin, Erez Berg and Andrea F. Young ()
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Ludwig Holleis: University of California, Santa Barbara
Tian Xie: University of California, Santa Barbara
Siyuan Xu: University of California, Santa Barbara
Haoxin Zhou: University of California, Santa Barbara
Caitlin L. Patterson: University of California, Santa Barbara
Archisman Panigrahi: Massachusetts Institute of Technology
Takashi Taniguchi: National Institute for Materials Science
Kenji Watanabe: National Institute for Materials Science
Leonid S. Levitov: Massachusetts Institute of Technology
Chenhao Jin: University of California, Santa Barbara
Erez Berg: Weizmann Institute of Science
Andrea F. Young: University of California, Santa Barbara

Nature, 2025, vol. 640, issue 8058, 355-360

Abstract: Abstract Magnetism typically arises from the effect of exchange interactions on highly localized fermionic wavefunctions in f- and d-atomic orbitals. By contrast, in rhombohedral multilayer graphene (RMG), magnetism—manifesting as spontaneous polarization into one or more spin and valley flavours1–7—originates from itinerant electrons near a Van Hove singularity. Here we show experimentally that the electronic entropy in this system indicates signatures typically associated with disordered local magnetic moments, unexpected for electrons in a fully itinerant metal. Specifically, we find a contribution ΔS ≈ 1 kB per charge carrier that begins at the Curie temperature and survives more than one order of magnitude in temperature. First-order phase transitions show an isospin ‘Pomeranchuk effect’ in which the fluctuating moment phase is entropically favoured over the nearby symmetric Fermi liquid8,9. Our results imply that, despite the itinerant nature of the electron wavefunctions, the spin and valley polarization of individual electrons is decoupled, a phenomenon typically associated with localized moments, as happens, for example, in solid 3He (ref. 10). Transport measurements, surprisingly, show a finite-temperature resistance minimum in the fluctuating moment regime, which we attribute to the interplay of fluctuating magnetic moments and electron–phonon scattering. Our results highlight the universality of soft isospin modes to two-dimensional flat-band systems.

Date: 2025
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DOI: 10.1038/s41586-025-08725-5

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