  Cuprate high temperature superconductors consist of two quasi-two-dimensional
(2D) substructures: CuO2 superconducting layers and charge reservoir layers.
The superconductivity is realized by charge transfer from the charge reservoir
layers into the superconducting layers without chemical dopants and defects
being introduced into the latter, similar to modulation-doping in semiconductor
superlattices of AlGaAs/GaAs. Inspired by this scheme, we have been searching
for high temperature superconductivity in ultrathin films of superconductors
epitaxially grown on semiconductor/oxide substrates since 2008. We have
observed interface enhanced superconductivity in both conventional and
unconventional superconducting films, including single atomic layer films of Pb
and In on Si substrates and single unit cell (UC) films of FeSe on SrTiO3 (STO)
substrates. The discovery of high temperature superconductivity with a
superconducting gap of ~20 meV in 1UC-FeSe/STO has stimulated tremendous
interest in superconductivity community, for it opens new avenue for both
raising superconducting transition temperature and understanding the pairing
mechanism of unconventional high temperature superconductivity. Here, we review
mainly the experimental progress on interface enhanced superconductivity in the
three systems mentioned above with emphasis on 1UC-FeSe/STO, studied by
scanning tunneling microscopy/spectroscopy, angle-resolved photoemission
spectroscopy and transport experiments. We discuss the roles of interfaces and
possible pairing mechanism inferred from those studies.
