Monolayer FeSe on SrTiO3 substrate has an enhanced superconducting transition temperature Tc than bulk FeSe. The superconducting samples can only be grown by molecular beam epitaxy with tens of hours’ post-growth annealing. We adjust the film stoichiometry by depositing additional small amounts of Fe atoms. The monolayers become superconducting after the Fe deposition without annealing, and show similar superconducting transition temperatures in transport measurements. The annealing process in essence removes Fe vacancies and the additional Fe deposition serves as a more efficient way. Through a complementary analysis of scanning transmission electron microscopy and in situ surface x-ray diffraction, we resolve the atomic-scale structure of the FeSe/SrTiO3 interface with a double TiO2 layer presented. ab initio calculations reveal a stronger tendency for electrons to transfer from an oxygen deficient SrTiO3 surface to FeSe when the double TiO2 layer is present. The interfacial coupling together with the stoichiometry of FeSe film has been well-accepted as necessary criteria for the enhanced superconductivity.

References:

K. Zou, S. Mandal, S. Albright, R. Peng, Y. Pu, D. Kumah, G. Simon, O. Dagdeviren, X. He, I. Božović, U.D. Schwarz, E.I. Altman, D. Feng, F.J. Walker, S. Ismail-Beigi, and C.H. Ahn, “Role of double TiO2 layers at the interface of FeSe/SrTiO3 superconductors”, Physical Review B, 93, 180506(R) (2016).

R. Peng, K. Zou, M. G. Han, S.D. Albright, H. Hong, C. Lau, H. C. Xu, Y. Zhu, F.J. Walker, C.H. Ahn, “Picoscale structural insight into superconductivity of monolayer FeSe/SrTiO3”, Science Advances, 6, 15, 4517 (2020). 

C. Liu, and K. Zou, “Tuning stoichiometry and its impact on superconductivity of monolayer and multilayer FeSe on SrTiO3”, Physical Review B, 101, 140502(R) (2020).