Key Factors for Insulator–Superconductor Transition in FeSe Thin Films by Electric Field

Abstract

Insulator-like FeSe epitaxial thin films were grown at three different growth conditions (i.e., at Fe flux rates = 2, 4, and 6 × 10 -6 Pa under a constant Se flux rate), and the superconducting properties induced by electric double-layer transistors (EDLTs) were investigated in relation to the crystal/defect structures of the FeSe channels. The maximum critical temperature (T c ) of 35 K was obtained for the FeSe channel grown at the intermediate Fe flux rate. The other EDLTs (i.e., grown at a higher and a lower Fe flux rates, respectively) exhibited lower T c of ≈20 K, which were in the underand the overdoped states, respectively, as confirmed by Hall effect measurements. The underdoped state of the channel grown at the higher rate is explained by electron traps at surface pits. This study revealed that high crystallinity and atomically flat surfaces are the key factors to achieving the optimum doping level and inducing the high T c in insulator-like FeSe EDLTs.