  It is well known that external magnetic fields and magnetic moments of
impurities both suppress superconductivity. Here, we demonstrate that their
combined effect enhances the superconductivity of a few atomic layer thick Pb
films grown on a cleaved GaAs(110) surface. A Ce-doped film, where
superconductivity is totally suppressed at zero-field, actually turns
superconducting when an external magnetic field is applied parallel to the
conducting plane. For films with Mn adatoms, the screening of the magnetic
moment by conduction electrons, i.e., the Kondo singlet formation, becomes
important. We found that the degree of screening can be reduced by capping the
Pb film with a Au layer, and observed the positive magnetic field dependence of
the superconducting transition temperature.
