"High-field quantum spin Hall effect in grapheme" |
| TYPE | Solid State Institute Seminar |
| Speaker: | Dr. Andrea Young |
| Affiliation: | Pappalardo Fellow in Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, U.S.A. |
| Date: | 23.06.2013 |
| Time: | 08:30 |
| 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. |