films*

The electrostatic doping of electrons into FeSe with the electric-double layer transistor (EDLT) structure enhances T_c of FeSe to 40 K. FeSe films with EDLT structure are also electrochemically etched by applying a gate voltage (V_g) at high temperature (>240 K). We previously suggested the existence of an electrochemical reaction layer at the surface of etched FeSe films which is responsible for the high T_c of 40 K [1]. If it is so, how T_c depends on V_g, might be complicated when compared with simple electrostatic doping.
We investigated the gate voltage dependence of T_c in electrochemically etched FeSe films. The T_c^zero value of the etched FeSe films with low gate voltages (V_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 (Nat. Mat. 14, 285 (2015)). Our results suggest that the origin of the increase in T_c is not electrostatic doping but rather the electrochemical reaction at the surface of the etched films [2].
[1] S. Kouno et al., Sci. Rep. 8 14731 (2018).
[2] N. Shikama et al., Appl. Phys. Express 13, 083006 (2020).