Strong-Magnetic-Field Magnon Transport in Monolayer Graphene

At high magnetic fields, monolayer graphene hosts competing phases\ndistinguished by their breaking of the approximate SU(4) isospin symmetry.\nRecent experiments have observed an even denominator fractional quantum Hall\nstate thought to be associated with a transition in the underlying isospin\norder from a spin-singlet charge density wave at low magnetic fields to an\nantiferromagnet at high magnetic fields, implying that a similar transition\nmust occur at charge neutrality. However, this transition does not generate\ncontrast in typical electrical transport or thermodynamic measurements and no\ndirect evidence for it has been reported, despite theoretical interest arising\nfrom its potentially unconventional nature. Here, we measure the transmission\nof ferromagnetic magnons through the two dimensional bulk of clean monolayer\ngraphene. Using spin polarized fractional quantum Hall states as a benchmark,\nwe find that magnon transmission is controlled by the detailed properties of\nthe low-momentum spin waves in the intervening Hall fluid, which is highly\ndensity dependent. Remarkably, as the system is driven into the\nantiferromagnetic regime, robust magnon transmission is restored across a wide\nrange of filling factors consistent with Pauli blocking of fractional quantum\nhall spin-wave excitations and their replacement by conventional ferromagnetic\nmagnons confined to the minority graphene sublattice. Finally, using devices in\nwhich spin waves are launched directly into the insulating charge-neutral bulk,\nwe directly detect the hidden phase transition between bulk insulating charge\ndensity wave and a canted antiferromagnetic phases at charge neutrality,\ncompleting the experimental map of broken-symmetry phases in monolayer\ngraphene.\n