EVIDENCE OF VORTICES ON THE INSULATING SIDE OF THE SUPERCONDUCTOR-INSULATOR TRANSITION

The magnetoresistance of ultrathin insulating films of Bi has been stu died with magnetic fields applied parallel and perpendicular to the pl ane of the sample. Deep in the strongly localized regime, the magnetor esistance is negative and independent of field orientation. As film th icknesses increase, the magnetoresistance becomes positive, and a diff erence between values measured in perpendicular and parallel fields ap pears, which is a linear function of the magnetic field and is positiv e. This is not consistent with the quantum interference picture. We su ggest that it is due to vortices present on the insulating side of the superconductor-insulator transition.