title: Coexisting massive and massless Dirac fermions in symmetry-broken bilayer graphene.
abstract: 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.
authors.affiliation: 1] Advanced Light Source, E. O. Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA [2] Department of Molecular Physics, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany.
authors.forename: Keun Su
authors.initials: KS
authors.lastname: Kim
authors.forename: Andrew L
authors.initials: AL
authors.lastname: Walter
authors.forename: Luca
authors.initials: L
authors.lastname: Moreschini
authors.forename: Thomas
authors.initials: T
authors.lastname: Seyller
authors.forename: Karsten
authors.initials: K
authors.lastname: Horn
authors.forename: Eli
authors.initials: E
authors.lastname: Rotenberg
authors.forename: Aaron
authors.initials: A
authors.lastname: Bostwick
references.citation: Science. 2010 Nov 5;330(6005):812-6
references.pmid: 20947726
references.citation: Science. 2006 Aug 18;313(5789):951-4
references.pmid: 16917057
references.citation: Science. 2010 May 21;328(5981):999-1002
references.pmid: 20489018
references.citation: Science. 2011 Aug 12;333(6044):860-3
references.pmid: 21836011
references.citation: Science. 2006 May 26;312(5777):1191-6
references.pmid: 16614173
references.citation: Phys Rev Lett. 2007 Dec 7;99(23):236809
references.pmid: 18233399
references.citation: Phys Rev Lett. 2007 Dec 21;99(25):256802
references.pmid: 18233543
references.citation: Nat Mater. 2008 Feb;7(2):151-7
references.pmid: 18059274
references.citation: Nat Mater. 2007 Mar;6(3):183-91
references.pmid: 17330084
references.citation: Phys Rev Lett. 2006 Mar 3;96(8):086805
references.pmid: 16606214
references.citation: Phys Rev Lett. 2009 Jan 9;102(1):015501
references.pmid: 19257205
references.citation: Proc Natl Acad Sci U S A. 2011 Jul 26;108(30):12233-7
references.pmid: 21730173
references.citation: Nat Nanotechnol. 2012 Jan 22;7(3):156-60
references.pmid: 22266634
references.citation: Phys Rev Lett. 2009 Dec 11;103(24):246804
references.pmid: 20366220
references.citation: Nature. 2007 Mar 1;446(7131):60-3
references.pmid: 17330039
references.citation: Phys Rev Lett. 2007 May 18;98(20):206802
references.pmid: 17677726