Charge and vortex Kosterlitz-Thouless transitions in Bi-superconducting mixed crystal thin films

Temperature (T) dependence of the sheet resistance (R-square) has been inve stigated on the c-axis oriented thin films of the (Bi2212/Bi2201) mixed cry stal with different molar fractions. The R-square-T superconducting charact eristic deteriorated with reduction of the Bi2212 fraction, and almost disa ppears at 48 mol% where a superconductor-to-insulator transition took place , with the resistance in the normal state, R-N, reaching 4.1 k Omega at 80 K. This R-N value is close to the universal quantum number, h/(2e)(2) congr uent to 6.5 k Omega predicted by the Kosterlitz-Thouless (KT) transition th eory. The R-square-T characteristics of the 48 mol% thin film can be elucid ated as a competitive process of KT transition brought about by charge or v ortex in the two-dimensional layer structure. (C) 1999 Elsevier Science B.V . All rights reserved.