title: Topological valley transport at bilayer graphene domain walls.
abstract: Electron valley, a degree of freedom that is analogous to spin, can lead to novel topological phases in bilayer graphene. A tunable bandgap can be induced in bilayer graphene by an external electric field, and such gapped bilayer graphene is predicted to be a topological insulating phase protected by no-valley mixing symmetry, featuring quantum valley Hall effects and chiral edge states. Observation of such chiral edge states, however, is challenging because inter-valley scattering is induced by atomic-scale defects at real bilayer graphene edges. Recent theoretical work has shown that domain walls between AB- and BA-stacked bilayer graphene can support protected chiral edge states of quantum valley Hall insulators. Here we report an experimental observation of ballistic (that is, with no scattering of electrons) conducting channels at bilayer graphene domain walls. We employ near-field infrared nanometre-scale microscopy (nanoscopy) to image in situ bilayer graphene layer-stacking domain walls on device substrates, and we fabricate dual-gated field effect transistors based on the domain walls. Unlike single-domain bilayer graphene, which shows gapped insulating behaviour under a vertical electrical field, bilayer graphene domain walls feature one-dimensional valley-polarized conducting channels with a ballistic length of about 400 nanometres at 4 kelvin. Such topologically protected one-dimensional chiral states at bilayer graphene domain walls open up opportunities for exploring unique topological phases and valley physics in graphene.
authors.affiliation: Department of Physics, University of California, Berkeley, California 94720, USA.
authors.forename: Long
authors.initials: L
authors.lastname: Ju
authors.affiliation: Department of Physics, University of California, Berkeley, California 94720, USA.
authors.forename: Zhiwen
authors.initials: Z
authors.lastname: Shi
authors.affiliation: Department of Physics, University of California, Berkeley, California 94720, USA.
authors.forename: Nityan
authors.initials: N
authors.lastname: Nair
authors.affiliation: Department of Physics, University of California, Berkeley, California 94720, USA.
authors.forename: Yinchuan
authors.initials: Y
authors.lastname: Lv
authors.affiliation: Department of Physics, University of California, Berkeley, California 94720, USA.
authors.forename: Chenhao
authors.initials: C
authors.lastname: Jin
authors.affiliation: Department of Physics, University of California, Berkeley, California 94720, USA.
authors.forename: Jairo
authors.initials: J
authors.lastname: Velasco
authors.affiliation: Department of Physics, University of California, Berkeley, California 94720, USA.
authors.forename: Claudia
authors.initials: C
authors.lastname: Ojeda-Aristizabal
authors.affiliation: Advanced Light Source Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.
authors.forename: Hans A
authors.initials: HA
authors.lastname: Bechtel
authors.affiliation: Advanced Light Source Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.
authors.forename: Michael C
authors.initials: MC
authors.lastname: Martin
authors.affiliation: 1] Department of Physics, University of California, Berkeley, California 94720, USA [2] Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA [3] Kavli Energy NanoSciences Institute at the University of California, Berkeley and the Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.
authors.forename: Alex
authors.initials: A
authors.lastname: Zettl
authors.affiliation: 1] Department of Physics, University of California, Berkeley, California 94720, USA [2] Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA [3] Kavli Energy NanoSciences Institute at the University of California, Berkeley and the Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.
authors.forename: James
authors.initials: J
authors.lastname: Analytis
authors.affiliation: 1] Department of Physics, University of California, Berkeley, California 94720, USA [2] Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA [3] Kavli Energy NanoSciences Institute at the University of California, Berkeley and the Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.
authors.forename: Feng
authors.initials: F
authors.lastname: Wang