Electrical transport in ultrathin films of a-bismuth at low temperatures and high magnetic fields near the superconductor -insulator transition.

摘要

Electrical transport properties of a-Bismuth deposited on a-Germanium underlayers have been measured. The properties of these films depend strongly on thickness. The thinnest films are insulating and electrons are strongly localized with an activated temperature dependence of the resistance. The hopping exponent decreases as a function of thickness in an approximately linear fashion, consistent with previous measurements. Measurements in a portion of this regime found that the response of the film becomes glass-like in the presence of high magnetic fields. Such observations are believed to be new and unique. As the thickness is further increased, films enter a regime where conduction is dominated by the weak localization mechanism. With very small increases in thickness, strong superconducting fluctuations become apparent. At the lowest temperatures the resistance becomes temperature independent. This metallic regime is believed to be intrinsic and not due to insufficient cooling of the carriers in the system. This is concluded from analysis of magnetoresistance data. This data shows a large region of negative magnetoresistance which changes upon cooling into the metallic regime. Systematics of this metallic behavior in magnetic field have been examined and also suggest that the effects are intrinsic. We suggest that given the success of finite-size scaling analysis at higher temperatures the system "thinks" it is approaching a quantum critical point, but the nucleation of the superconducting (and near the presumed transition the insulating) phase is interrupted. We present various possible explanations of the data. These include vortex and charge depinning, and various dissipation mechanisms.