Bilayer graphene (BLG) at the charge neutrality point (CNP) is strongly
susceptible to electronic interactions, and expected to undergo a phase
transition into a state with spontaneous broken symmetries. By systematically
investigating a large number of singly- and doubly-gated bilayer graphene (BLG)
devices, we show that an insulating state appears only in devices with high
mobility and low extrinsic doping. This insulating state has an associated
transition temperature Tc~5K and an energy gap of ~3 meV, thus strongly
suggesting a gapped broken symmetry state that is destroyed by very weak
disorder. The transition to the intrinsic broken symmetry state can be tuned by
disorder, out-of-plane electric field, or carrier density

A. H. MacDonald

B. Standley

Bolotin

C. N. Lau

Castro

D. Smirnov

F. Zhang

Freitag

J. Velasco

L. Jing

M. Bockrath

Martin

Mayorov

Rossi

Velasco

W. Bao

Zhang

Zhao

English

arXiv

At the charge neutrality point, bilayer graphene (BLG) is strongly susceptible to electronic interactions and is expected to undergo a phase transition to a state with spontaneously broken symmetries. By systematically investigating a large number of single-and double-gated BLG devices, we observe a bimodal distribution of minimum conductivities at the charge neutrality point. Although σmin is often approximately 2–3 e^2/h (where e is the electron charge and h is Planck’s constant), it is several orders of magnitude smaller in BLG devices that have both high mobility and low extrinsic doping. The insulating state in the latter samples appears below a transition temperature T_c of approximately 5 K and has a T = 0 energy gap of approximately 3 meV. Transitions between these different states can be tuned by adjusting disorder or carrier density

Bao, Wenzhong

Velasco, Jairo, Jr.

Zhang, Fan

Jing, Lei

Standley, Brian

Smirnov, Dmitry

Bockrath, Marc

MacDonald, Allan H.

Lau, Chun Ning

Caltech Authors - Main

Evidence for a spontaneous gapped state in ultraclean bilayer graphene


            At the charge neutrality point, bilayer graphene (BLG) is strongly susceptible to electronic interactions and is expected to undergo a phase transition to a state with spontaneously broken symmetries. By systematically investigating a large number of single-and double-gated BLG devices, we observe a bimodal distribution of minimum conductivities at the charge neutrality point. Although
            σ
            min
            is often approximately 2–3 
            e
            2
            /
            h
            (where
            e
            is the electron charge and
            h
            is Planck’s constant), it is several orders of magnitude smaller in BLG devices that have both high mobility and low extrinsic doping. The insulating state in the latter samples appears below a transition temperature
            T
            
              c
            
            of approximately 5 K and has a
            T
             = 0 energy gap of approximately 3 meV. Transitions between these different states can be tuned by adjusting disorder or carrier density.
          </jats:p

Wenzhong Bao

Jairo Velasco

Fan Zhang

Lei Jing

Brian Standley

Dmitry Smirnov

Marc Bockrath

Allan H. MacDonald

Chun Ning Lau

Crossref

Proceedings of the National Academy of Sciences

http://authors.library.caltech.edu/33340/1/Bao2012p19170P_Natl_Acad_Sci_Usa.pdf

Minimum Conductivity and Evidence for Phase Transitions in Ultra-clean
  Bilayer Graphene

Minimum Conductivity and Evidence for Phase Transitions in Ultra-clean Bilayer Graphene

Abstract

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