Lecturer: Prof. Andrea Young
Affiliation: University of California, Santa Barbara
Abstract:
Low-dimensional electronic systems have
traditionally been obtained by electrostatically
confining electrons, either in heterostructures
or in intrinsically nanoscale materials such as
nanowires. Recently, a new method has
emerged with the recognition that gapped
symmetry-protected topological (SPT) phases
can host robust surface states that remain
gapless as long as the relevant global symmetry
remains unbroken. The nature of the charge
carriers in SPT surface states is intimately tied
to the symmetry of the bulk, resulting in one-
and two-dimensional electronic systems with
novel properties such as the locking of spin and
momentum. I will describe our recent
experimental realization of such helical states
on the edge of a graphene flake subjected to
very large magnetic fields. In contrast to its
time-reversal-symmetric cousin, the graphene
quantum spin Hall state is protected by a
symmetry of planar spin rotations that
emerges as electron spins in a half-filled
Landau level are polarized by the applied field.
The properties of the resulting helical edge
states can be modulated by balancing the
applied field against an intrinsic
antiferromagnetic instability, which tends to
spontaneously break the spin-rotation
symmetry. In the resulting canted
antiferromagnetic state, we observe transport
signatures of gapped edge states, which
constitute a new kind of one-dimensional
electronic system with a tunable bandgap and
an associated spin texture.