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 500 nm forming a lattice of constrictions connected by islands of the film. At temperatures below the superconducting critical temperature of the islands, the magnetoresistance oscillations with a period of one flux ( h / 2 e ) per lattice cell have been clearly observed. However, in the critical region between the temperature of Berezinskii–Kosterlitz–Thouless 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 / 2 e and h / 4 e oscillations. The observation of h / 4 e 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.