Origin of the insulating state in exfoliated high-Tc two-dimensional atomic crystals

We present the results of an optical spectroscopic study of two- dimensional atomic crystals of the hightemperature superconductor Bi2Sr2CaCu2O8+delta. Our measurements reveal a pronounced suppression of the optical conductivity sigma(1)(omega) with thickness. Using an effective medium approximation, we interpret this in terms of an insulating surface layer. The surface layer explains the insulating behavior previously observed in exfoliated Bi2Sr2CaCu2O8+delta and has implications -for future studies and potential applications of these materials.