Probing Goldstone spin wave excitations through a quantum Hall anti-ferromagnet in bilayer graphene

The Landau levels and flat bands of twisted van der Waals heterostructures support a phenomenon known as quantum Hall ferromagnetism, where magnetic orders spontaneously develop in the spin or pseudospin sectors of a non-magnetic material due to strong Coulomb interactions. Quantum hall magnets support a variety of interesting low-energy collective excitations that reveal the nature of the ground states. In this talk, I will describe an all-electrical approach to obtain the dispersion relation ω(k) of spin wave excitations in bilayer graphene using transport devices that integrate a Fabry-Pérot cavity. Our measurements at the n=0 Landau level show evidence of gapless, linearly dispersing spin wave excitations, thereby providing direct evidence for an canted anti-ferromagnetic order with in-plane rotational symmetry. This technique is particularly suited to probe magnetic states formed under challenging experimental conditions such as low temperature and/or in a magnetic field. We anticipate its usage in studying other collective excitations of symmetry-broken ground states in van der Waals materials.  

Fu et al, Phys. Rev. X 11, 021012 (2021)

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