"High-field quantum spin Hall effect in grapheme"

TYPESolid State Institute Seminar
Speaker:Dr. Andrea Young
Affiliation:Pappalardo Fellow in Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, U.S.A.
Location:Solid State Auditorium(Entrance)
Remark:Host: Assistant Professor Ophir Auslaender
Abstract:The quantum spin Hall (QSH) effect is a two dimensional electronic phase characterized by
an excitation gap in the bulk but gapless, helical boundary states. Since its original
discovery in HgCdTe quantum wells, the QSH effect has become nearly synonymous with
the time-reversal-invariant two-dimensional topological insulator. I will describe recent
experiments in which we demonstrate a QSH in both mono- and bilayer graphene at charge
neutrality and high magnetic fields. In contrast to the traditional context, the existence of
our QSH phase depends on a spin-rotation symmetry and the absence of strong spin orbit
coupling. Using large in-plane magnetic fields, we drive a transition from a spin-unpolarized insulating
phase to a spin-polarized metallic phase with ~2e^2/h conductance
(4e^2/h for bilayer), in which we observe the nonlocal transport signatures of the
QSHE. Simultaneous capacitance measurements, which probe the bulk, show that
throughout the transition and into the QSH regime, the bulk gap never closes. We
understand this transition as resulting from a breaking of the spin-rotation symmetry via an
instability towards a canted antiferromagnetic state. Because the transition is due to a
change in symmetry, rather than topology, the gap does not close in the bulk, but only on
the edge, and we observe signatures of gapped, spin textured edge states that have no
experimental analog in traditional time-reversal symmetry based topological insulators.