Discrete Superconducting Phases in FeSe-Derived Superconductors

A general feature of unconventional superconductors is the existence of a superconducting dome in the phase diagram. Here we report a series of discrete superconducting phases in the simplest iron-based superconductor, FeSe thin flakes, by continuously tuning the carrier concentration through the intercalation of Li and Na ions with a solid ionic gating technique. Such discrete superconducting phases are robust against the substitution of 20% S for Se, but they are vulnerable to the substitution of 2% Cu for Fe, highlighting the importance of the iron site being intact. The superconducting phase diagram for FeSe derivatives is given, which is distinct from that of other unconventional superconductors.

Figure 1

(a) Schematic structure of the solid ionic gating device. From the bottom to the top: Silver back gate layer, finely polished Li/Na-ion substrate, pure or doped FeSe thin flake, 100 nm ${\mathrm{SiO}}_2$, and four electrodes. The gate voltage $V_G$ is applied at low temperature ($<155\mathrm{K}$), where all the ions in the substrate are frozen in place. (b) Representative optical image of a fabricated device. Shadow masks are used here to eliminate any contamination from the photoresist or developer. (c) Atomic force microscope (AFM) topographic image of the dashed rectangular area shown in (b). (d) Cross-sectional profile of the thin flake along the red line in (c), giving the thickness of the flake $t=17.5\mathrm{nm}$, which corresponds to about 31 FeSe monolayers. The thickness of the samples used in the present study ranges from 5 to 38 nm.

Figure 2

(a) Resistance of the Li-intercalated FeSe flake (17.5 nm) as a function of temperature. This panel covers the underdoped regime with the normal-state resistance monotonically decreasing upon gating. Colored dashed lines highlight transition temperatures where the $T_c$’s (in this work, we use $T_c^{\text{onset}}$ as $T_c$) of several curves coincide. $T_{c1}$, $T_{c2}$, and $T_{c3}$ are SC phases at 8, 36, and 44 K, respectively. (b) Resistance of another 10 nm FeSe flake in the overdoped regime with its normal-state resistance gradually increasing upon gating. (c) and (d) Color contour plots of the derivatives of the $R-T$ curves in (a) and (b). We rescaled the curves in (b) at 80 K before differentiating to see the SC transitions more clearly.

Figure 3

(a) and (b) Resistance of Na-intercalated FeSe flakes as a function of temperature, similar to those of Fig. 2. The sample thicknesses are (a) 18 nm and (b) 8.5 nm. Vertical broken lines highlight transition temperatures where the $T_c$’s of different curves coincide. $T_{c2}$ is observed throughout the entire doping region before the sample goes into the insulating phase. (c) and (d) Color contour plots of the derivatives of the $R-T$ curves in (a) and rescaled in (b).

Figure 4

(a) and (b) The gating dependence of $R-T$ curves for ${\mathrm{FeSe}}_{0.8}{\mathrm{S}}_{0.2}$ (34 nm thick) and ${\mathrm{Fe}}_{0.98}{\mathrm{Cu}}_{0.02}\mathrm{Se}$ (21.5 nm thick). (c) and (d) Their respective $dR/dT$ color contour plots. Note that the $T_{c2}^{\prime }$ of ${\mathrm{Li}}_x$$\mathrm{Fe}({\mathrm{Se}}_{0.8}{\mathrm{S}}_{0.2})$ may not necessarily correspond to $T_{c2}$ in ${\mathrm{Li}}_x\mathrm{FeSe}$. The inset in (b) expands the SC onset region.

Figure 5

Phase diagram of FeSe-derived superconductors. The red lines denote the discrete SC phases of metal-intercalated FeSe superconductors, as represented by ${\mathrm{Li}}_x\mathrm{FeSe}$. The different Fermi surface topologies of pristine and doped FeSe are superimposed. The yellow area on the right represents the insulating phase in the heavily overdoped region.