The role of mesoscopic disorder in determining the character of the field-induced insulating regime of amorphous ultrathin films

This work documents the conditions under which the field-tuned behavior of quench condensed films grown on relatively thick layers of a-Sb is essentially identical to that found for amorphous InOx and polycrystalline TiNx films. The electrical transport properties of a series of amorphous Bi (a-Bi) films of different thicknesses, grown with a 14.67 Å thick underlayer of amorphous Sb (a-Sb), were studied in perpendicular and parallel magnetic fields. A magnetoresistance (MR) peak was found in insulating films in both perpendicular and parallel magnetic fields. In all insulating films, Arrhenius type conduction was also found over all ranges of magnetic field. Neither behavior is found for a-Bi films grown on top of thinner a-Sb underlayers, which were substantially smoother. These observations, together with a quantitative analysis of film roughness profiles, highlight the role of mesoscopic scale thickness fluctuations in nucleating superconducting clusters or islands that apparently lead to the MR peak and Arrhenius conduction. This implies that the nature of the disorder plays a major role in determining the character of the insulating regime.