Tunable metal–insulator transition in double-layer graphene heterostructures

Disorder-induced Anderson localization usually causes conducting materials to become insulating at low temperature. Graphene is a notable exception. But by increasing the carrier density in one graphene layer, a metal–insulator transition can be induced in an isolated second layer stacked above it. 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 , 2 , 3 . Observed in a multitude of materials, including damaged graphene and its disordered chemical derivatives 4 , 5 , 6 , 7 , 8 , 9 , 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.