CRITICAL CURRENTS AND CRITICAL FIELDS IN COMPOSITE AL-GE FILMS

Lee and Stroud have predicted that the critical current I(c)(B) in a c omposite superconducting material should displav a constant value in m oderately high magnetic fields, giving rise to a 'plateau region'. Cri tical current measurements on granular metallic Al-Ge films, located c onsiderably above the metal-insulator transition (MIT), display well d efined plateaus, and these measurements confirm most of the Lee-Stroud predictions. In contrast, films located just above the MIT exhibit co ntinuously decreasing critical currents with increasing magnetic field s. These results are explained using a tunnelling junction model propo sed by Peterson and Ekin and extended by Papa and Altshuler. From the fitting results, a simple picture, involving both models, emerges, whi ch explains the behaviour of I(c) versus magnetic field in pressed pow dered YBCO samples and in Ag-sheathed Bi-based HTC wires, as well as i n the Al-Ge films.