Tunable metal-insulator transition in double-layer graphene heterostructures

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 approximate to 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.