Collective edge modes near the onset of a graphene
quantum spin Hall state
Graphene subject to a strong, tilted magnetic
field exhibits an
insulator-metal transition tunable by
tilt-angle, which is attributed to the
transition from a canted antiferromagnetic (CAF)
to a ferromagnetic (FM) bulk
state at filling factor \nu=0. We develop a
theoretical description for the
spin and valley edge textures in the two phases,
and the implied evolution in
the nature of edge modes through the transition.
In particular, we show that
the CAF has gapless neutral modes in the bulk,
but supports gapped charged
edge modes constructed by the binding of a
vortex (meron) in the bulk state to
a spin twist at the edge. The energy gap of this
edge mode is therefore
dictated by the bulk spin stiffness. At the
transition to the FM state
where the latter vanishes, the charged edge
modes become gapless and are
smoothly connected to the helical edge modes of
the FM state. We further
discuss possible experimental consequences.