Observation of Double-Dome Superconductivity in Potassium-Doped FeSe Thin Films

We report on the emergence of two disconnected superconducting domes in alkali-metal potassium- (K-)doped FeSe ultrathin films grown on graphitized SiC(0001). The superconductivity exhibits hypersensitivity to K dosage in the lower-$T_{\mathrm{c}}$ dome, whereas in the heavily electron-doped higher-$T_{\mathrm{c}}$ dome it becomes spatially homogeneous and robust against disorder, supportive of a conventional Cooper-pairing mechanism. Furthermore, the heavily K-doped multilayer FeSe films all reveal a large superconducting gap of $\sim 14\mathrm{meV}$, irrespective of film thickness, verifying the higher-$T_{\mathrm{c}}$ superconductivity only in the topmost FeSe layer. The unusual finding of a double-dome superconducting phase is a step towards the mechanistic understanding of superconductivity in FeSe-derived superconductors.

Figure 1

(a) STM topographic image of K-doped $\mathrm{FeSe}/\mathrm{SiC}(0001)$ films ($10\mathrm{nm}\times 10\mathrm{nm}$, $V=1.0\mathrm{V}$, $I=50\mathrm{pA}$). (b) Atomically resolved topography measured in K-free region ($2\mathrm{nm}\times 2\mathrm{nm}$, $V=2\mathrm{mV}$, $I=100\mathrm{pA}$), marked by the dashed square in (a). (c) Differential conductance $dI/dV$ spectra of FeSe films before (black curve) and after (red curve) a surface dose of 0.205 ML K. The two triangles denote the edges of the accidental gap between the electron and hole pockets around the $\mathrm{\Gamma }$ point. The tunneling gap is set at $V=0.5\mathrm{V}$, $I=100\mathrm{pA}$. The lock-in bias modulation has a magnitude of 10 meV.

Figure 2

(a)–(e) Topographies [(a) $5\mathrm{nm}\times 5\mathrm{nm}$. (b)–(e) $30\mathrm{nm}\times 30\mathrm{nm}$] and (f)–(j) $dI/dV$ spectra of multilayer $\mathrm{FeSe}/\mathrm{SiC}(0001)$ films with varying doses of K, as indicated. Blue arrows denote the superconducting gap edges or coherence peaks. The absence of an apparent $E_F$-symmetric gap in (h) shows the full suppression of superconductivity in FeSe by an intermediate dose of K. Tunneling conditions: (a) $V=-10\mathrm{mV}$, $I=100\mathrm{pA}$; (b) $V=4.0\mathrm{V}$, $I=20\mathrm{pA}$; (c) $V=4.0\mathrm{V}$, $I=10\mathrm{pA}$; (d) $V=1.0\mathrm{V}$, $I=10\mathrm{pA}$; (e) $V=3.0\mathrm{V}$, $I=20\mathrm{pA}$; (f) $V=10\mathrm{mV}$, $I=100\mathrm{pA}$; (g)–(j) $V=20\mathrm{mV}$, $I=100\mathrm{pA}$.

Figure 3

(a) $dI/dV$ spectrum normalized by dividing the raw $dI/dV$ spectrum in Fig. 2 by its background, which was extracted from a cubic fit to the conductance for $|V|>6\mathrm{mV}$. The red-shaded region characterizes the superconductivity-induced spectral weight loss $\delta$ near $E_F$. (b) Electron doping level $x$ dependence of $\delta$ (black triangles) or superconductivity, confirming two disconnected superconducting domes ($L$-SC and $H$-SC phases) in the electron-doped FeSe phase diagram.

Figure 4

(a) Spatial and (b) film thickness dependence of $dI/dV$ spectra acquired on heavily electron-doped multilayer FeSe films ($x\sim 0.103$). Red and blue dashes show the approximate energy positions of the two energy-scale superconducting gaps, respectively. The film thickness of FeSe is indicated in a unit of triple layer (TL). (c) A series of $dI/dV$ spectra acquired along a 10-nm trajectory on heavily electron-doped single layer $\mathrm{FeSe}/\mathrm{SiC}$ films ($x\sim 0.103$). Red dashes indicate the energy positions of the superconducting gap. Setpoint: $V=20\mathrm{mV}$, $I=100\mathrm{pA}$.