Thickness dependence of superconductivity and superconductor–insulator transition in ultrathin FeSe films on SrTiO ₃ (001) substrate

Abstract

Interface-enhanced high-temperature superconductivity in one unit-cell (UC)\nFeSe film on SrTiO3(001) (STO) substrate has recently attracted much attention\nin condensed matter physics and material science. Here, by ex situ transport\nmeasurements, we report on the superconductivity in FeSe ultra-thin films with\ndifferent thickness on STO substrate. We find that the onset superconducting\ntransition temperature (Tc) decreases with increasing film thickness of FeSe,\nwhich is opposite to the behavior usually observed in traditional\nsuperconductor films. By systematic post-annealing of 5 UC FeSe films, we\nobserve an insulator to superconductor transition, which is accompanied with a\nsign change of the dominated charge carriers from holes to electrons at low\ntemperatures according to the corresponding Hall measurement.\n