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 high- temperature superconductor Bi₂Sr₂CaCu₂O_8+δ. Our measurements reveal a pronounced suppression of the optical conductivity σ₁(ω) 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 Bi₂Sr₂CaCu₂O_8+δ and has implications for future studies and potential applications of these materials.

Physical Review B Rapid Communications 90, 081402 (2014)