  Heterostructure based interface engineering has been proved an effective
method for finding new superconducting systems and raising superconductivity
transition temperature (TC). In previous work on one unit-cell (UC) thick FeSe
films on SrTiO3 (STO) substrate, a superconducting-like energy gap as large as
20 meV, was revealed by in situ scanning tunneling microscopy/spectroscopy
(STM/STS). Angle resolved photoemission spectroscopy (ARPES) further revealed a
nearly isotropic gap of above 15 meV, which closes at a temperature of ~ 65 K.
If this transition is indeed the superconducting transition, then the 1-UC FeSe
represents the thinnest high TC superconductor discovered so far. However, up
to date direct transport measurement of the 1-UC FeSe films has not been
reported, mainly because growth of large scale 1-UC FeSe films is challenging
and the 1-UC FeSe films are too thin to survive in atmosphere. In this work, we
successfully prepared 1-UC FeSe films on insulating STO substrates with
non-superconducting FeTe protection layers. By direct transport and magnetic
measurements, we provide definitive evidence for high temperature
superconductivity in the 1-UC FeSe films with an onset TC above 40 K and a
extremely large critical current density JC ~ 1.7*106 A/cm2 at 2 K. Our work
may pave the way to enhancing and tailoring superconductivity by interface
engineering.
