Percolative behavior of the superconductive transition of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ films

The superconductive transition of c-axis-oriented ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ thin films, as characterized by measurements of the temperature dependence of the magnetic penetration depth, is found to occur below the Ginzburg-Landau mean-field transition. The divergence of the penetration depth at the transition is in good quantitative agreement with the predictions of a percolation model.