Negative magnetoresistance and quantum oscillations in ultrathin perforated PtSi films

We study the magnetic field dependence of the resistance of ultrathin superconducting PtSi films perforated with a square pattern of holes with a period of 500nm forming a lattice of constrictions connected by islands of the film. At temperatures below the superconducting critical temperature of the islands, the magneto resistance oscillations with a period of one flux (h/2e) per lattice cell have been clearly observed. However, in the critical region between the temperature of Berezinskii-Kosterlitz-Thoutess transition and the mean field BCS transition temperature the oscillations are superimposed on a background of negative magnetoresistance in weak magnetic field. Careful analysis of the shape of the oscillations shows that they are a superposition of h/2e and h/4e oscillations. The observation of h/4e period of Aharonov-Bohm oscillations simultaneously with a negative magnetoresistance supports the prediction that correlation effects can produce a negative Josephson coupling in a disordered superconductor. (c) 2006 Elsevier B.V. All rights reserved.