  All non-interacting two-dimensional electronic systems are expected to
exhibit an insulating ground state. This conspicuous absence of the metallic
phase has been challenged only in the case of low-disorder, low density,
semiconducting systems where strong interactions dominate the electronic state.
Unexpectedly, over the last two decades, there have been multiple reports on
the observation of a state with metallic characteristics on a variety of
thin-film superconductors. To date, no theoretical explanation has been able to
fully capture the existence of such a state for the large variety of
superconductors exhibiting it. Here we show that for two very different
thin-film superconductors, amorphous indium-oxide and a single-crystal of
2H-NbSe2, this metallic state can be eliminated by filtering external
radiation. Our results show that these superconducting films are extremely
sensitive to external perturbations leading to the suppression of
superconductivity and the appearance of temperature independent, metallic like,
transport at low temperatures. We relate the extreme sensitivity to the
theoretical observation that, in two-dimensions, superconductivity is only
marginally stable.
