Electric-field-induced superconductivity in electrochemically etched ultrathin FeSe films on SrTiO ₃ and MgO

Among the recently discovered iron-based superconductors, ultrathin films of FeSe grown on SrTiO ₃ substrates have uniquely evolved into a high-transition-temperature (T c) superconducting material. 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.