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 ${\mathrm{FeSe}}_{1-x}{\mathrm{Te}}_x$ and ${\mathrm{FeSe}}_{1-y}{\mathrm{S}}_y$ films with $0\le x\le 0.4$ and $0\le y\le 0.25$. 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.

Figure 1

(a)–(e) Shift of the temperature dependence of the resistance $R$ of the etched ${\mathrm{FeSe}}_{1-x}{\mathrm{Te}}_x$ and ${\mathrm{FeSe}}_{1-y}{\mathrm{S}}_y$ thin films as the etching process was repeated for $x=y=0,x=0.2,x=0.4,y=0.06$, and $y=0.25$, respectively. Arrows indicate the shift over the etching processes. (f) The temperature dependence of the temperature derivative of resistance of the etched FeSe.

Figure 2

Estimated carrier densities $n$ multiplied by thickness of the surface-doped layer $d$ for $x=0.2$ [37] and $y=0.25$ films.

Figure 3

The Te- and S-content dependence of $T_{\mathrm{c}}$ in undoped [32, 33] and e-doped ${\mathrm{FeSe}}_{1-x}{A}_x$ ($A=\mathrm{Te},\mathrm{S}$) films on LAO. Those of undoped ${\mathrm{FeSe}}_{1-x}{\mathrm{Te}}_x$ films on ${\mathrm{CaF}}_2$ [31, 43] and those of bulk undoped [50, 51, 52] and intercalated samples [24, 27, 28] are also plotted. The dotted lines are guides for the eye.

Figure 4

Temperature dependence of the normalized resistance of FeSe films on LAO, ${\mathrm{CaF}}_2$, and STO substrates with an EDLT structure before and after the electrochemical etching/doping with $V_{\mathrm{g}}=2.5\mathrm{V}$.

Figure 5

Obtained phase diagram of the undoped [32, 33] and the e-doped ${\mathrm{FeSe}}_{1-x}{\mathrm{Te}}_x$ and ${\mathrm{FeSe}}_{1-y}{\mathrm{S}}_y$ films. $T_{\mathrm{mag}}$ represents the magnetic transition temperature in undoped ${\mathrm{FeSe}}_{1-y}{\mathrm{S}}_y$ films [39]. The gap-opening-temperature $T_{\mathrm{\Delta }}$ of the single-layer ${\mathrm{FeSe}}_{1-x}{\mathrm{Te}}_x/{\mathrm{SrTiO}}_3$ [53] are also plotted.