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 ${\mathrm{LaAlO}}₃$ with Se substituted by isovalent Te and S. Our method enables transport measurements of e-doped FeSe. Electron doping by applying a gate voltage of 5 V increases ${T}_{\mathrm{c}}$ of the ${\mathrm{FeSe}}_{1\ensuremath{-}x}{\mathrm{Te}}_x$ and ${\mathrm{FeSe}}_{1\ensuremath{-}y}{\mathrm{S}}_y$ films with $0\ensuremath{\le}x\ensuremath{\le}0.4$ and $0\ensuremath{\le}y\ensuremath{\le}0.25$. Applying both positive and negative chemical pressures monotonically suppresses ${T}_{\mathrm{c}}$ of the e-doped FeSe. ${T}_{\mathrm{c}}$ and its evolution with chemical pressure is very different between undoped and e-doped FeSe, which is considered to be related to the difference in change of density of states at Fermi energy as a function of chemical pressure in the two systems. These results might be one of the manifestations of the possible difference of superconductivity mechanisms in these systems.