Positive and negative chemical pressure effects investigated in electron-doped FeSe films with an electric-double-layer structure

We investigated chemical pressure effects in electron-doped (e-doped) FeSe by fabricating an electric-double-layer structure with single crystalline FeSe films on LaAlO${}_3$ with Se substituted by isovalent Te and S. Our method enables transport measurements of e-doped FeSe. Electron doping by applying gate voltage of 5 V increases $T_{\mathrm{c}}$ of the FeSe${}_{1-x}$Te${}_x$ and FeSe${}_{1-y}$S${}_y$ films with $0\le x\le 0.4$ and $0\le y\le 0.25$, while the e-doped $x=0.5$ film showed lower $T_{\mathrm{c}}$ than that of the undoped one. Both positive and chemical pressure suppress $T_{\mathrm{c}}$ of the e-doped FeSe. The obtained superconducting phase diagram in the e-doped samples is rather different from that in undoped samples. This might suggest that the superconductivity mechanism is different between undoped and e-doped systems. Alternatively, this is possibly explained by the absence of the nematic order in e-doped samples.