  In this study, we investigated the gate voltage dependence of $T_{\mathrm c}$
in electrochemically etched FeSe films with an electric-double layer transistor
structure. The $T_{\mathrm c}^{\mathrm {zero}}$ value of the etched FeSe films
with a lower gate voltage ($V_{\mathrm g}$ = 2.5 and 3.3 V) reaches 46 K, which
is the highest value among almost all reported values from the resistivity
measurements except for the data by Ge et al. This enhanced $T_{\mathrm c}$
remains unchanged even after the discharge process, unlike the results for
electrostatic doping without an etching process. Our results suggest that the
origin of the increase in $T_{\mathrm c}$ is not electrostatic doping but
rather the electrochemical reaction at the surface of an etched films.

Superconductivity