LETTERS PUBLISHED ONLINE: 9 OCTOBER 2011 | DOI: 10.1038/NPHYS2114 Tunable metal–insulator transition in double-layer graphene heterostructures L. A. Ponomarenko 1 , A. K. Geim 1,2 , A. A. Zhukov 2 , R. Jalil 2 , S. V. Morozov 1,3 , K. S. Novoselov 1 , I. V. Grigorieva 1 , E. H. Hill 2 , V. V. Cheianov 4 , V. I. Fal’ko 4 , K. Watanabe 5 , T. Taniguchi 5 and R. V. Gorbachev 2 * Disordered conductors with resistivity above the resistance quantum h/e 2 should exhibit an insulating behaviour at low temperatures, a universal phenomenon known as a strong (Anderson) localization 1–3 . Observed in a multitude of materials, including damaged graphene and its disordered chemical derivatives 4–10 , Anderson localization has not been seen in generic graphene, despite its resistivity near the neutrality point reaching ≈h/e 2 per carrier type 4,5 . It has remained a puzzle why graphene is such an exception. Here we report a strong localization and the corresponding metal–insulator transition in ultra-high-quality graphene. The transition is controlled externally, by changing the carrier density in another graphene layer placed at a distance of several nm and decoupled electrically. The entire behaviour is explained by electron–hole puddles that disallow localization in standard devices but can be screened out in double- layer graphene. The localization that occurs with decreasing rather than increasing disorder is a unique occurrence, and the reported double-layer heterostructures presents a new experimental system that invites further studies. Resistivity values ≈h/e 2 indicate that the electron mean free path l is shorter than the Fermi wavelength λ F , so that quantum interference becomes a dominant feature in electron diffusion, leading to Anderson localization in the absence of phase-breaking processes at low temperatures (T ). The scope of this phenomenon extends beyond electronic systems—into optical and acoustic phenomena as well 1–3 —but not generic graphene, which remains metallic at liquid-helium T (refs 4,5) and exhibits only a weak T dependence that can be explained by phonons and thermally excited carriers 11 . Earlier theoretical studies have suggested that Dirac electrons can evade localization for certain types of disorder 3,12–15 , with the extreme example being graphene subjected to a smooth Coulomb potential 16,17 . However, for generic disorder that involves scattering between the two graphene valleys, the localization is expected to be unavoidable 3,18,19 . Experiments do not show this. In this Letter, we describe a double-layer electronic system made of two closely-spaced but electrically isolated graphene monolayers sandwiched in boron nitride. In the following, the two layers in the double layer graphene (DLG) heterostructure are referred to as the studied and control layers. At low doping n C in the control layer, the studied layer exhibits the standard behaviour with a minimum metallic conductivity of ∼4e 2 /h. However, for n C > 10 11 cm −2 , the resistivity ρ of the studied layer diverges near the neutrality point (NP) at T < 70 K. This divergence can be suppressed by a 1 School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK, 2 Manchester Centre for Mesoscience and Nanotechnology, Manchester M13 9PL, UK, 3 Institute for Microelectronics Technology, 142432 Chernogolovka, Russia, 4 Physics Department, University of Lancaster, Lancaster LA1 4YB, UK, 5 National Institute for Materials Science, 1-1 Namiki, Tsukuba, 305-0044, Japan. *e-mail:blizza@gmail.com. small perpendicular field B < 0.1 T, which indicates that this is an interference effect rather than a gap opening. We attribute the metal–insulator transition (MIT) to the recovery of an intrinsic behaviour such that graphene exhibits Anderson localization if its ρ reaches values of ≈h/e 2 per carrier type. Normally, this intrinsic MIT is obscured by charge inhomogeneity in the form of electron– hole puddles 20–24 . Within each puddle, graphene is sufficiently away from the NP and remains metallic. Then, resistivity of the percolating electron–hole system with leaking p–n boundaries 16,17 assumes a value of ∼h/e 2 with little T dependence (conceptually this value has little in common with the similar value required for Anderson localization) 23,24 . The control layer can screen out the fluctuating background potential and suppress electron–hole puddles, revealing the intrinsic properties at the NP. This reconciles the metallic behaviour normally observed in graphene with the localization expected for large ρ and supports the idea that the minimum conductivity that tends to assume values close to 4e 2 /h is due to electron–hole puddles 23,24 . The studied devices were fabricated by sandwiching two graphene monolayers with thin hexagonal-BN crystals. In a multistep procedure, described in the Supplementary Information, a graphene monolayer was transferred onto a 20–30 nm thick BN crystal that was first prepared on top of an oxidized Si wafer. Then, the graphene was covered with another BN crystal (spacer), which was followed by transfer of the second graphene layer. Both layers were shaped into multiterminal devices aligned above each other and having separate electrical contacts (Fig. 1a). Individual steps were similar to those described in refs 25,26 but the whole fabrication process involved three dry transfers and alignments, four nonconsecutive rounds of electron-beam lithography, three rounds of plasma etching and two separate metal depositions. The resulting DLG heterostructures are schematically shown in Fig. 1a (for images, see Supplementary Information). We made several such devices with channel widths of 1–2 μm. They exhibited μ of 30 –120 × 10 3 cm 2 V −1 s −1 and little chemical doping. The bottom layer encapsulated in BN always had higher μ and changed little after exposure to air 26 whereas the quality of the top layer gradually decayed. For this particular study, we employed three multiterminal devices with sufficiently thick BN spacers to avoid any detectable tunnel current between graphene layers (<0.1 nA). The spacers had thicknesses d ≈ 4, 12 and 16 nm. All the devices exhibited a similar MIT behaviour, although the insulating state was much more pronounced for devices with smaller d and higher μ, as described below. 958 NATURE PHYSICS | VOL 7 | DECEMBER 2011 | www.nature.com/naturephysics © 20 11 M acmillan Publishers Limited. All rights reserved.