Tunable symmetry breaking and helical edge transport in a graphene quantum spin Hall state

Applying a very large magnetic field to charge-neutral monolayer graphene produces a symmetry-protected quantum spin Hall state with helical edge states whose properties can be modulated by balancing the applied field against an intrinsic antiferromagnetic instability. Graphene on the edge In the search for new electronic states made robust against disturbances by their topological features, Pablo Jarillo-Herrero and colleagues have identified graphene edge states that result from strong interactions between electrons. In contrast to well-studied topological insulators in which time reversal symmetry breaking has an essential role, these newly discovered graphene states are protected by symmetry rules. The novel electronic states, which separate electrons by their spin, appear when graphene is subjected to a large magnetic field angled with respect to the plane. The dependence on electron–electron interactions makes it possible to control the features with a voltage, revealing a fundamentally new electronic system with tunable electronic band gap and associated spin properties. Low-dimensional electronic systems have traditionally been obtained by electrostatically confining electrons, either in heterostructures or in intrinsically nanoscale materials such as single molecules, nanowires and graphene. Recently, a new method has emerged with the recognition that symmetry-protected topological (SPT) phases 1 , 2 , which occur in systems with an energy gap to quasiparticle excitations (such as insulators or superconductors), 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. For example, time reversal symmetry endows the massless charge carriers on the surface of a three-dimensional topological insulator with helicity, fixing the orientation of their spin relative to their momentum 3 , 4 . Weakly breaking this symmetry generates a gap on the surface 5 , resulting in charge carriers with finite effective mass and exotic spin textures 6 . Analogous manipulations have yet to be demonstrated in two-dimensional topological insulators, where the primary example of a SPT phase is the quantum spin Hall state 7 , 8 . Here we demonstrate experimentally that charge-neutral monolayer graphene has a quantum spin Hall state 9 , 10 when