  Among the recently discovered iron-based superconductors, ultrathin films of
FeSe grown on SrTiO3 substrates have uniquely evolved into a high
superconducting-transition-temperature (TC) material. The mechanisms for the
high-TC superconductivity are ongoing debate mainly with the superconducting
gap characterized with in-situ analysis for FeSe films grown by bottom-up
molecular-beam epitaxy. Here, we demonstrate the alternative access to
investigate the high-TC superconductivity in ultrathin FeSe with top-down
electrochemical etching technique in three-terminal transistor configuration.
In addition to the high-TC FeSe on SrTiO3, the electrochemically etched
ultrathin FeSe transistor on MgO also exhibits superconductivity around 40 K,
implying that the application of electric-field effectively contributes to the
high-TC superconductivity in ultrathin FeSe regardless of substrate material.
Moreover, the observable critical thickness for the high-TC superconductivity
is expanded up to 10-unit-cells under applying electric-field and the
insulator-superconductor transition is electrostatically controlled. The
present demonstration implies that the electric-field effect on both conduction
and valence bands plays a crucial role for inducing high-TC superconductivity
in FeSe.
