Thickness dependence of superconductivity and superconductor-insulator transition in ultrathin FeSe films on SrTiO3(001) substrate

Interface-enhanced high-temperature superconductivity in a one-unit-cell (UC) FeSe film on SrTiO3 (001) (STO) substrate has recently attracted much attention in condensed matter physics and material science. Here, using ex situ transport measurements, we report on the superconductivity in FeSe ultrathin films with different thicknesses on STO substrate. We find that the onset superconducting transition temperature (T-c) decreases with increasing film thickness of FeSe, which is opposite to the behavior usually observed in traditional superconductor films. By systematic post-annealing of 5 UC FeSe films, we observe an insulator-superconductor transition, which is accompanied by a sign change of the dominant charge carriers from holes to electrons at low temperatures according to the corresponding Hall measurement.