Electric-field-induced superconductivity in electrochemically etched ultrathin FeSe films on SrTiO3 and MgO

Electric-field-induced superconductivity in samples of ultrathin FeSe grown on SrTiO 3 and MgO substrates shows that the superconductivity is not an interfacial effect but is rather related to a charge imbalance of electrons and holes. Among the recently discovered iron-based superconductors 1 , 2 , 3 , ultrathin films of FeSe grown on SrTiO 3 substrates have uniquely evolved into a high-transition-temperature ( T c ) superconducting material 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 . The mechanisms for the high- T c superconductivity are under debate, with the superconducting gap mainly characterized with in situ analysis for FeSe films grown by molecular beam epitaxy. Here, we investigate the high- T c superconductivity in ultrathin FeSe using an alternative top-down electrochemical etching technique in a three-terminal transistor configuration. In addition to the high-temperature superconductivity in FeSe on SrTiO 3 , the electrochemically etched ultrathin FeSe transistor on MgO also exhibits superconductivity around 40 K, implying that the application of an electric field effectively contributes to the high- T c superconductivity in ultrathin FeSe regardless of substrate material. Moreover, the observable critical thickness for the high- T c superconductivity is expanded up to 10 unit cells under an applied electric field and the insulator–superconductor transition is electrostatically controlled. The present demonstration implies that the modification of charge imbalance of holes and electrons by the electric-field effect plays a crucial role in inducing high- T c superconductivity in FeSe-based electric double-layer transistors.