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Excitonic nature of magnons in a quantum Hall ferromagnet


Magnons enable the transfer of a magnetic moment or spin over macroscopic distances. In quantum Hall ferromagnets, it has been predicted1 that spin and charge are entangled, meaning that any change in the spin texture modifies the charge distribution. As a direct consequence of this entanglement, magnons should carry an electric dipole moment. Here we report evidence of this electric dipole moment in a graphene quantum Hall ferromagnet2,3 using a Mach–Zehnder interferometer. As magnons propagate across the insulating bulk, their electric dipole moment modifies the Aharonov–Bohm flux through the interferometer, affecting both phase and visibility of the interference pattern. In particular, we relate the phase shift to the sign of this electric dipole moment and the loss of visibility to the flux of emitted magnons, and we show that the magnon emission is a Poissonian process. Finally, we probe the emission energy threshold of the magnons for transient states, between ν = 0 and ν = 1, and link them to the emergence of the gapless mode predicted in the canted-antiferromagnetic phase at charge neutrality4,5. The ability to couple the spin degree of freedom to an electrostatic potential is a property of quantum Hall ferromagnets that could be promising for spintronics.

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Fig. 1: Transmission and reflection of magnons at a p–n junction interface.
Fig. 2: Magnon detection with an MZI.
Fig. 3: Phase shift and decoherence as a function of the injected current into an MZI.
Fig. 4: Magnon properties around νB = 1 and when approaching νB = 0 or νB = 2.

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We warmly thank H.-S. Sim, J.-Y. Lee, M. O. Goerbig, C.-L. Huang, N. Wei and A. M. Donald for enlightening discussions, as well as P. Jacques for technical support. We thank W. Dumnernpanich for his help in fabrication. This work was funded by the ERC starting grant COHEGRAPH 679531 (P. Roulleau), by the EMPIR project SEQUOIA 17FUN04 co-financed by the participating states and the EU’s Horizon 2020 programme (P. Roulleau) and by ‘Investissements d’Avenir’ LabEx PALM (ANR-10-LABX-0039-PALM) (Project ZerHall) (F.D.P.).

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A.A., M.J., P.B. and P. Roulleau performed the experiment with help from F.D.P. and P. Roche. A.A., P.B., M.J., F.D.P. and P. Roulleau analysed and discussed the data with help from P. Roche. T.J. and P. Roulleau developed the theoretical model. T.T. and K.W. provided the boron nitride layers. M.J. fabricated the device with inputs from P.B., A.A., F.D.P. and P. Roulleau. A.A., F.D.P., P. Roche and P. Roulleau wrote the manuscript with inputs from all the authors. P. Roulleau designed and supervised the project.

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Correspondence to P. Roulleau.

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Peer review information Nature Physics thanks Petr Stepanov, So Takei and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary Discussion and Figs. 1–18.

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Assouline, A., Jo, M., Brasseur, P. et al. Excitonic nature of magnons in a quantum Hall ferromagnet. Nat. Phys. 17, 1369–1374 (2021).

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