Superconductivity in immiscible Nb–Cu nanocomposite films

We report the superconducting properties of immiscible Nb–Cu nanocomposite films with varying compositions. The microstructure of the films revealed the presence of phase separated, closely spaced, nano-grains of Nb and Cu whose sizes changed marginally with composition. In all films we observe two resistive transitions. Analysis of the superconducting phase transition from temperature dependences of DC resistivity and AC susceptibility and comparison of the superconducting transition temperatures with that in nanoparticles of pure Nb with different particle sizes permit us to make a conclusion about a possible establishment of a global phase coherence in Nb–Cu system. The temperature variation of the critical current fits well with the Ambegaokar-Baratoff theory and this agreement suggests that our thick Nb–Cu films possibly behave like a random 3D network of Josephson junctions.