Gapless Spin Wave Transport Through a Quantum Canted-Antiferromagnet

The Landau levels of graphene and magic-angle van der Waals heterostructures support quantum Hall ferromagnetism, where magnetism spontaneously develops in the spin and pseudospin degrees of freedom due to strong interactions. A quantum Hall ferromagnet can support a rich variety of gapped and gapless spin wave excitations, some of which may be useful as quantum information carriers. In this work, we demonstrate 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 to resonantly select discrete wave vectors of the spin wave. We report on the observation of gapless, linearly dispersed spin wave excitations in the ν = 0 Landau level of bilayer graphene. The gapless spin wave propagates with a high group velocity of several tens of km/s and maintains long-distance phase coherence. This result provides direct experimental evidence for a predicted easy-plane, canted anti-ferromagnetic order in this material and lays the foundation for the exploration of spin superfluidity. The resonant cavity technique we developed can be generalized to investigate other collective excitation of spin and pseudospin symmetry-broken ground states.