Enhanced superconductivity induced by several-unit-cells diffusion in an FeTe/FeSe bilayer heterostructure

Unlike monolayer Fe-Chalcogenide (Fe-$Ch$)$/\mathrm{SrTi}{\mathrm{O}}_3$ (STO), which possesses the potential for high-temperature superconductivity (HTS), a regular Fe-$Ch$ thin film grown on a non-STO substrate by the pulsed laser deposition method shows totally different superconducting behavior and a different mechanism. Although regular Fe-$Ch$ thick films grown on $\mathrm{Ca}{\mathrm{F}}_2$ generally show the highest superconducting transition temperature ($T_{\mathrm{c}}$) compared with any other substrates, the disappearance of superconductivity always takes place when the thickness of the Fe-$Ch$ film is reduced to a critical value ($\sim 20\mathrm{nm}$ for Fe-Se and $\sim 30\mathrm{nm}$ for Fe-Se-Te) with the reason still under debate. Here, we report an enhanced $T_{\mathrm{c}}\approx 17.6\mathrm{K}$ in a 7-nm-FeTe/7-nm-FeSe bilayer heterostructure grown on $\mathrm{Ca}{\mathrm{F}}_2$ substrate. Generally, the Fe-$Ch$ film on $\mathrm{Ca}{\mathrm{F}}_2$ is supposed to be one order of magnitude greater in thickness to achieve similar performance. Hall measurements manifest the dominant nature of hole-type carriers in the films in this work, which is similar to the case of a pressurized bulk FeSe single crystal, while in sharp contrast to heavily electron-doped HTS Fe-$Ch$ systems. According to the electron energy loss spectroscopy results, we observed direct evidence of nanoscale phase separation in the form of a fluctuation of the Fe-$L_3/L_2$ ratio near the FeTe/FeSe interface. In detail, a several-unit-cell-thick Fe(Se,Te) diffusion layer shows a higher Fe-$L_3/L_2$ ratio than either an FeTe or an FeSe layer, indicating low Fe $3d$ electron occupancy, which is, to some extent, consistent with the hole-dominant scenario obtained from the Hall results. It also implies a possible relationship between the state of Fe $3d$ electron occupancy and the enhanced $T_{\mathrm{c}}$ in this work. Our work clarifies the importance of the FeTe/FeSe interface in reviving the superconductivity in Fe-$Ch$ ultrathin films, contributing to a more unified understanding of unconventional Fe-$Ch$ superconductivity.

Figure 1

(a) Temperature dependence of the normalized resistance $(R/R^{25\mathrm{K}})$ at self-field for #FT-300 and #FT-r.t. in this work from 2 to 25 K. Additionally, the results from a 8 nm FeSe thin film [55] (black triangles), a 127 nm FeSe thin film [55] (magenta diamonds), and a 200 nm $\mathrm{FeS}{\mathrm{e}}_{0.5}\mathrm{T}{\mathrm{e}}_{0.5}$ thin film [56] (olive stars) are added on the same coordinates for comparison. The $T_{\mathrm{c}}^{\mathrm{onset}}/T_{\mathrm{c}}^{\mathrm{zero}}$ values for each sample are also given. (b) $R/R^{20\mathrm{K}}-T$ measurements of #FT-300 under external fields up to 8 T along the $c$ axis. The inset shows the plots of $B-T_{\mathrm{c}}$ . The upper critical field $\left(B_{\mathrm{c}2}\right)$ is estimated from the linear extrapolation of $T_{\mathrm{c}}^{\mathrm{mid}}$. The results from a 127 nm FeSe thin film [55] are also plotted for comparison.

Figure 2

Temperature dependence of the Hall coefficient $R_{\mathrm{H}}$ (left $y$ axis) and zero-field normalized resistance $(R/R^{300\mathrm{K}})$ (right $y$ axis) for (a) #FT-300 and (b) #FT-r.t. Insets: Hall resistivity ${\rho }_{xy}$ vs ${\mu }_{0}H$ at different temperatures.

Figure 3

(a) XRD patterns for #FT-300 (red) and #FT-r.t. (blue) in the range $2\theta =10–{55}^{\circ }$. The indices (hkl) represent the reflections of the FeSe (brown), Fe(Se,Te) (red), and FeTe (magenta) phases, respectively. The Ag phase in #FT-r.t. comes from the residual silver paste after transport measurements. Dashed lines are guides for the eye to mark the locations of the Fe-Chalcogenide peaks. (b) Enlarged $2\theta$ interval near the $\beta$-FeSe (001) peak. The $y$ axis is set to a logarithmic scale. The locations of the peaks are marked by dashed lines. The Fe(Se,Te) phase is found coexisting with FeSe and FeTe in #FT-300, while it is almost absent in #FT-r.t. (c),(d) Dark-field high-resolution STEM micrographs focusing on the FeTe/FeSe transition region from a cross-sectional viewpoint for (c) #FT-300 and (d) #FT-r.t. The two micrographs are exactly identical in scale after normalization. The corresponding EDS-mapping results are shown on the right side of each STEM image, showing the distribution of Ca (green), Se (cyan), Te (magenta), Fe (yellow), and O (blue) elements. The EDS line scans of the same region show the relative variation of Fe, Se, and Te elements in the form of atomic ratio percentages. For reference only, the relative variation trend of the $c$-axis lattice parameter of each Fe-chalcogenide layer was measured and is plotted on the micrograph in the form of $c/c^{\mathrm{FeSe}}$ vs atomic layer ($c^{\mathrm{FeSe}}$ refers to the $c$-axis lattice parameter of the first FeSe layer adjacent to the CaSe interlayer). A much broader Fe(Se,Te) diffusion layer is found in #FT-300. Red dashed lines are only guides for the eye.

Figure 4

(a) Schematic diagram of the electronic excitation process shows how the Fe-$L_3$ and Fe-$L_2$ edges appear in EELS. (b) An example of the Fe-$L_3$ (cyan) and Fe-$L_2$ (yellow) edges. The shaded areas are the integrals of $L_3$ and $L_2$ white lines.

Figure 5

Core-loss EELS results acquired at room temperature for the two samples in this work. (a) A HAADF image of #FT-300 with a dashed rectangle indicating the area where spectra were collected. The HAADF image of #FT-r.t. was taken under the same conditions (not shown here). The inset shows the position-dependent Fe-$L_3/L_2$ ratio in #FT-300 (red circles) and #FT-r.t. (black squares) along the EELS scan direction. A relatively high Fe-$L_3/L_2$ ratio in #FT-300 in the Fe(Se,Te) region is the only difference. The regions denoted as Fe-O, FeTe, Fe(Se,Te), and FeSe layers are defined based on the EDS results on #FT-300. The same definition is utilized in #FT-r.t. despite the much sharper transition. The typical areas of different regions are indicated by horizontal lines on the HAADF image as well as in the core-loss EELS mapping shown in (b). (c) Separate EELS spectra for the four regions in the two samples (red circles: #FT-300; black squares: #FT-r.t.). A dashed oval in the region of Fe(Se,Te) highlights the only difference in a white-line feature. (d) A magnified range of the EELS spectra showing the energy shift of the Fe-$L_3$ edge in the Fe(Se,Te) region of #FT-300 compared with that of #FT-r.t. from the same area. The error bar is 0.15 eV. (e) White-line Fe-$L_3/L_2$ ratios for the four regions in #FT-300 are presented in the form of horizontal lines. The “$L_3/L_2$ ratio–$3d$ occupancy” results from Graetz et al. [80] and Sparrow et al. [91] are reproduced as solid diamonds for reference. The dashed arrows are a guide for the eye to show the empirical correlation between the white-line ratio and the $3d$ occupancy of transition-metal oxides ($\mathrm{TM}{\mathrm{O}}_x$) that was summarized in Ref. [80].