Negative magnetoresistance in fractal Pb thin films on Si(111)

Using a low temperature method, the authors have prepared atomically flat Pb ultrathin films on $Si(111)-7×7$ surface. Room temperature annealing of the films results in a percolation morphology with fractal vacancy islands where the Si substrate is exposed. The fractal film with a nominal thickness of 23 ML exhibits enhanced onset superconducting transition temperature of $7.0K$ and negative magnetoresistance with wide magnetoresistance terrace under perpendicular magnetic field when the film is in superconducting state. They attribute the phenomena to the coexistence of two superconducting phases in this fractal film.