Direct measurement of sheet resistance R□ in cuprate systems:: Evidence of a fermionic scenario in a metal-insulator transition

The metal-insulator transition (MIT) has been studied in Ba0.9Nd0.1CuO2+x/CaCuO2 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 k Omega universal quantum resistance. All data confirm the assumption that each CaCuO2 layer forms a 2D superconducting sheet within the superconducting block, which can be described as weak-coupled equivalent sheets in parallel.