  Recent experiments showed that thinning gallium, iron selenide and 2H
tantalum disulfide to single/several monoatomic layer(s) enhances their
superconducting critical temperatures. Here, we characterize these
superconductors by extracting the absolute values of the London penetration
depth, the superconducting energy gap, and the relative jump in specific heat
at the transition temperature from their self-field critical currents. Our
central finding is that the enhancement in transition temperature for these
materials arises from the opening of an additional superconducting gap, while
retaining a largely unchanged bulk superconducting gap. Literature data reveals
that ultrathin niobium films similarly develop a second superconducting gap.
Based on the available data, it seems that, for type-II superconductors, a new
superconducting band appears when the film thickness becomes smaller than the
out-of-plane coherence length. The same mechanism may also be the cause of
enhanced interface superconductivity.
