Charge carriers in bilayer graphene are widely believed to be massive Dirac fermions that have a bandgap tunable by a transverse electric field. However, a full transport gap, despite its importance for device applications, has not been clearly observed in gated bilayer graphene, a long-standing puzzle. Moreover, the low-energy electronic structure of bilayer graphene is widely held to be unstable towards symmetry breaking either by structural distortions, such as twist, strain, or electronic interactions that can lead to various ground states. Which effect dominates the physics at low energies is hotly debated. Here we show both by direct band-structure measurements and by calculations that a native imperfection of bilayer graphene, a distribution of twists whose size is as small as ∼0.1, is sufficient to generate a completely new electronic spectrum consisting of massive and massless Dirac fermions. The massless spectrum is robust against strong electric fields, and has a unusual topology in momentum space consisting of closed arcs having an exotic chiral pseudospin texture, which can be tuned by varying the charge density. The discovery of this unusual Dirac spectrum not only complements the framework of massive Dirac fermions, widely relevant to charge transport in bilayer graphene, but also supports the possibility of valley Hall transport.
Bibliographical noteFunding Information:
This work and ALS were supported by the US Department of Energy, Office of Sciences under Contract No. DE-AC02-05CH11231. K.S.K. acknowledges support by an NRF Grant funded by the Korean Government (NRF-2011-357-C00022). A.L.W. acknowledges support by the Max Planck Society. L.M. acknowledges support by Grant PA00P2-136420 from the Swiss National Science Foundation (SNSF). T.S. was supported by the DFG in the framework of the Priority Program 1459 ‘Graphene’. We thank F. Speck, M. Ostler and F. Fromm for assistance during sample preparation, and J. Denlinger for assistance during measurement.
All Science Journal Classification (ASJC) codes
- Materials Science(all)
- Condensed Matter Physics
- Mechanics of Materials
- Mechanical Engineering