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Visualizing orbital magnetism in electron doped rhombohedral multilayer graphene
Authors:
Owen I. Sheekey,
Trevor B. Arp,
Benjamin A. Foutty,
Ruoxi Zhang,
Tixuan Tan,
Ludwig F. W. Holleis,
Yi Guo,
Sandesh S. Kalantre,
Canxun Zhang,
Mark Zakharyan,
David Gong,
Aidan Keough,
Youngjoon Choi,
Ysun Choi,
Siyuan Xu,
Tian Xie,
Ben Hodder Alexander,
Marisa Hocking,
Qingrui Cao,
Martin E. Huber,
Takashi Taniguchi,
Kenji Watanabe,
Chenhao Jin,
Etienne Lantagne-Hurtubise,
Aaron Sharpe
, et al. (2 additional authors not shown)
Abstract:
Electron doped rhombohedral multilayer graphene at high displacement field features an exceptionally flat band minimum with near-ideal quantum geometry. Experiments in this regime observe the formation of a 'quarter metal,' in which the electron liquid condenses into a single spin- and valley flavor. Remarkably, recent experiments have found a zero resistance state in the same region of the densit…
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Electron doped rhombohedral multilayer graphene at high displacement field features an exceptionally flat band minimum with near-ideal quantum geometry. Experiments in this regime observe the formation of a 'quarter metal,' in which the electron liquid condenses into a single spin- and valley flavor. Remarkably, recent experiments have found a zero resistance state in the same region of the density- and displacement-field-tuned parameter space, attributed to the formation of a chiral superconductor from an orbitally ferromagnetic normal state. Here, we use nanoSQUID-on-tip magnetometry to map the orbital magnetization of electron-doped rhombohedral graphene devices ranging in thickness between 3 and 15 layers. Magnetization within the quarter metal phases peaks at finite density, consistent with concentration of the Berry curvature in a finite-momentum 'ring of fire'. Correlating transport and local magnetometry data in a superconducting tetralayer sample reveals a finite orbital ferromagnetic moment, providing direct evidence of valley polarization in the superconducting ground state. We further show that widely observed stochastic switching of the resistivity in both metallic and superconducting regimes arises from a density-tuned sign change in the valley-resolved total magnetic moment. This leads to the formation of metastable magnetic domains under typical gate control sequences and can also be harnessed for electric-field controlled switching of the magnetization across the entire device. Finally, high resolution measurements of the magnetization across a superconducting transition allow us to put an upper bound on the 'condensation magnetization' of 0.1 Bohr magneton per carrier, placing a strong quantitative restriction on theoretical models for ferromagnetic superconductivity.
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Submitted 4 August, 2026; v1 submitted 28 May, 2026;
originally announced May 2026.
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Superconductivity and quantized anomalous Hall in rhombohedral graphene
Authors:
Youngjoon Choi,
Ysun Choi,
Marco Valentini,
Caitlin L. Patterson,
Ludwig F. W. Holleis,
Owen I. Sheekey,
Hari Stoyanov,
Xiang Cheng,
Takashi Taniguchi,
Kenji Watanabe,
Andrea F. Young
Abstract:
Inducing superconducting correlations in chiral edge states is predicted to generate topologically protected zero energy modes with exotic quantum statistics. Experimental efforts to date have focused on engineering interfaces between superconducting materials typically amorphous metals and semiconducting quantum Hall or quantum anomalous Hall (QAH) systems. However, the interfacial disorder inher…
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Inducing superconducting correlations in chiral edge states is predicted to generate topologically protected zero energy modes with exotic quantum statistics. Experimental efforts to date have focused on engineering interfaces between superconducting materials typically amorphous metals and semiconducting quantum Hall or quantum anomalous Hall (QAH) systems. However, the interfacial disorder inherent in this approach can prevent the formation of isolated topological modes. An appealing alternative is to use low-density flat band materials where the ground state can be tuned between intrinsic superconducting and quantum anomalous Hall states using only the electric field effect. However, quantized transport and superconductivity have not been simultaneously achieved. Here, we show that rhombohedral tetralayer graphene aligned to a hexagonal boron nitride substrate hosts a quantized anomalous Hall state at superlattice filling $ν=-1$ as well as a superconducting state at $ν-3.5$ at zero magnetic field. Remarkably, gate voltage can also be used to actuate nonvolatile switching of the chirality in the quantum anomalous Hall state, allowing, in principle, arbitrarily reconfigurable networks of topological edge modes in locally gated devices. Thermodynamic compressibility measurements further reveal a topologically ordered fractional Chern insulator at $ν=2/3$-also stable at zero magnetic field-enabling proximity coupling between superconductivity and fractionally charged edge modes. Finally, we show that, as in rhombohedral bi- and trilayers, integrating a transition metal dichalcogenide layer to the heterostructure nucleates a new superconducting pocket, while leaving the topology of the $ν=-1$ quantum anomalous Hall state intact. Our results pave the way for a new generation of hybrid interfaces between superconductors and topological edge states in the low-disorder limit.
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Submitted 15 April, 2025; v1 submitted 22 August, 2024;
originally announced August 2024.
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Superconductivity and spin canting in spin-orbit proximitized rhombohedral trilayer graphene
Authors:
Caitlin L. Patterson,
Owen I. Sheekey,
Trevor B. Arp,
Ludwig F. W. Holleis,
Jin Ming Koh,
Youngjoon Choi,
Tian Xie,
Siyuan Xu,
Evgeny Redekop,
Grigory Babikyan,
Haoxin Zhou,
Xiang Cheng,
Takashi Taniguchi,
Kenji Watanabe,
Chenhao Jin,
Etienne Lantagne-Hurtubise,
Jason Alicea,
Andrea F. Young
Abstract:
Graphene and transition metal dichalcogenide flat-band systems show similar phase diagrams, replete with magnetic and superconducting phases. An abiding question has been whether magnetic ordering competes with superconductivity or facilitates pairing. The advent of crystalline graphene superconductors enables a new generation of controlled experiments to probe the microscopic origin of supercondu…
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Graphene and transition metal dichalcogenide flat-band systems show similar phase diagrams, replete with magnetic and superconducting phases. An abiding question has been whether magnetic ordering competes with superconductivity or facilitates pairing. The advent of crystalline graphene superconductors enables a new generation of controlled experiments to probe the microscopic origin of superconductivity. For example, recent studies of Bernal bilayer graphene show a dramatic increase in the observed domain and critical temperature $T_c$ of superconducting states in the presence of enhanced spin-orbit coupling; the mechanism for this enhancement, however, remains unclear. Here, we show that introducing spin-orbit coupling in rhombohedral trilayer graphene (RTG) via substrate proximity effect generates new superconducting pockets for both electron and hole doping, with maximal $T_c\approx$ 300mK three times larger than in RTG encapsulated by hexagonal boron nitride alone. Using local magnetometry and thermodynamic compressibility measurements, we show that superconductivity straddles an apparently continuous transition between a spin-canted state with a finite in-plane magnetic moment and a state with complete spin-valley locking. This transition is reproduced in our Hartree-Fock calculations, where it is driven by the competition between spin-orbit coupling and the carrier-density-tuned Hund's interaction. Our experiment suggests that the enhancement of superconductivity by spin-orbit coupling is driven not by a change in the ground state symmetry or degeneracy but rather by a quantitative change in the canting angle. These results align with a recently proposed mechanism for the enhancement of superconductivity in spin-orbit coupled rhombohedral multilayers, in which fluctuations in the spin-canting order contribute to the pairing interaction.
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Submitted 19 August, 2024;
originally announced August 2024.