The metal-insulator transition (MIT) has been studied in ${\mathrm{Ba}}_{0.9}{\mathrm{Nd}}_{0.1}{\mathrm{CuO}}_{2+x}/{\mathrm{CaCuO}}_2$ ultrathin cuprate structures. Such structures allow for the direct measurement of the 2D sheet resistance $R_{\square }$, eliminating ambiguity in the definition of the effective thickness of the conducting layer in high temperature superconductors. The MIT occurs at room temperature for experimental values of $R_{\square }$ close to the $25.8\mathrm{k}\Omega$ universal quantum resistance. All data confirm the assumption that each ${\mathrm{CaCuO}}_2$ layer forms a 2D superconducting sheet within the superconducting block, which can be described as weak-coupled equivalent sheets in parallel.

• Received 25 October 2006

DOI:https://doi.org/10.1103/PhysRevLett.98.036401