Previous STM/STS study shows square-like 1 × 1 structure with the in-plane lattice constants a = b = 3.82 Å and 20 meV superconducting-like gap for the 1-UC FeSe films on STO substrates 7. However, direct evidence of T C above 60 K for FeSe films on conductive substrates and systematic transport studies of FeSe films on insulating substrates are still absent. Following, the T C above 40 K in 1-UC FeSe films on insulating STO has been demonstrated by direct transport measurements and Meissner effect 7. In previous work, in situ scanning tunneling microscopy/spectroscopy (STM/STS) 12 and angle resolved photoemission spectroscopy (ARPES) 13, 14, 15 detections on 1-UC thick FeSe films on Nb-doped STO (conductive STO) substrates revealed a superconducting energy gap as large as 20 meV and above 15 meV (closing at a temperature of 65 ± 5 K) respectively, indicating a possible T C higher than 60 K. In the meanwhile, heterostructure based interface engineering has been proved an effective method for raising T C due to the enhancement of electron-phonon coupling 10 or epitaxial strain 11. FeSe, with the simplest structure and less toxicity in iron-based superconductors (among all five families), has become one of attractive materials but the T C is relatively low in bulk state (~8 K) 9. Iron-based superconductors has triggered great interest 1 because of the high transition temperature 2, 3, ultra-high critical magnetic field 4, 5, 6 and potential applications as a group of high T C superconductors 7, 8.

Srtio3 lattice constant