Ultrathin topological insulators don't seem to behave the way they are supposed to. Instead of becoming insulators due to hybridization of the two closely-spaced Dirac surfaces, the system seems to remain metallic at all temperatures and for all positions of the chemical potential. There is also the surprising appearance of linear magnetoresistance, which becomes increasingly prominent as one approaches the Dirac point.
In a recent paper, Debaleena Nandi and I addressed these problems in a joint experimental-theoretical paper. We show how both of these surprises can be explained as a product of a long-range-correlated disorder potential arising from charged impurities. Along the way, we have suggested a new mechanism for linear magnetoresistance and shown how increasing disorder can turn an insulator into a metal.
The work is joint with Bert Halperin and with the groups of Amir Yacoby and Philip Kim at Harvard, and with the group of Jagadeesh Moodera at MIT.