Origin of the insulating state in exfoliatedhigh-Tctwo-dimensional atomic crystals

An optical spectroscopic study of exfoliated two-dimensional atomic crystals of Bi${}_{2}$Sr${}_{2}$CaCu${}_{2}$O${}_{8+\ensuremath{\delta}}$ reveals a pronounced suppression of optical conductivity with thickness. Through detailed analysis, the authors interpret this finding as the formation of an insulating surface layer.