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

TL;DR

This study explores how chemical and electric pressures influence superconductivity in electron-doped FeSe films, revealing that both pressures suppress $T_c$ and suggesting different mechanisms in doped versus undoped systems.

Contribution

It introduces a novel electric-double-layer method to measure transport in e-doped FeSe and compares the effects of chemical and electric pressures on $T_c$ and phase diagrams.

Findings

Both positive and chemical pressure suppress $T_c$ in e-doped FeSe.

Electron doping increases $T_c$ up to a point, then decreases it.

Superconducting phase diagram differs from undoped FeSe, indicating different mechanisms.

Abstract

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 $0 \leq x \leq 0.4$ and $0 \leq y \leq 0.25$, 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.