We prepared one-unit-cell (1-UC) thick FeSe films on insulating SrTiO3 substrates with non-superconducting FeTe
protection layers by molecular beam epitaxy for ex situ studies. By direct transport and magnetic measurements,
we provide definitive evidence for high temperature superconductivity in the 1-UC FeSe films with an onset ?C
above 40 K and an extremely large critical current density ?C∼1.7×106 A/cm2
at 2 K, which are much higher
than ?C∼8 K and ?C∼104 A/cm2
for bulk FeSe, respectively. Our work may pave the way to enhancing and
tailoring superconductivity by interface engineering.