  A surprisingly strong variation of resistance with perpendicular magnetic
field, and a peak in the resistance vs. field, R(B) has been found in
insulating films of a sequence of homogeneous, quench-condensed films of
amorphous Bi undergoing a thickness-tuned superconductor-insulator transition.
Isotherms of magnetoresistance, rather than resistance, vs. field were found to
cross at a well-defined magnetic field higher than the field corresponding to
the peak in R(B). For all values of B, R(T) was found to obey an Arrhenius
form. At the crossover magnetic field the prefactor became equal to the quantum
resistance of electron pairs, h/4e^2, and the activation energy returned to its
zero field value. These observations suggest that the crossover is the
signature of a quantum phase transition between two distinct insulating ground
states, tuned by magnetic field.
