Induced Superconducting Transition in Ultra-Thin Iron-Selenide Films by a Mg-Coating Process

Abstract

Binary Iron selenide (FeSe) thin films have been widely studied for years to unveil the high temperature superconductivity in iron-based superconductors. However, the origin of superconducting transition in this unconventional system is still under debate and worth deep investigations. In the present work, the transition from insulator to superconductor was achieved in non-superconducting FeSe ultrathin films (~8 nm) grown on calcium fluoride substrates via a simple in-situ Mg-coating by a pulsed laser deposition technique. The Mg-coated FeSe film with an optimized amount of Mg exhibited a superconducting critical temperature as 9.7 K and an upper critical field as 30.9 T. Through systematic characterizations on phase identification, carrier transport behavior and high-resolution microstructural features, the revival of superconductivity in FeSe ultrathin films is mostly attributed to the highly crystallized FeSe and extra electron doping received from external Mg-coating process. Although the top few FeSe layers are incorporated with Mg, most FeSe layers are intact and protected by a stable magnesium oxide layer. This work provides a new strategy to induce superconductivity in FeSe films with non-superconducting behavior, which might contribute to a more comprehensive understanding of iron-based superconductivity and the benefit to downstream applications such as magnetic resonance imaging, high-field magnets and electrical cables.

Keywords: iron-based superconductor; pulsed laser deposition; thin film; transmission electron microscopy.

Figure 1

X-ray diffraction (XRD) θ-2θ patterns of #UFM0, #UFM1, #UFM3 and #UFM5. (hkl) signs represent the diffraction peaks of β-FeSe, while asterisk signs are originated from CaF₂ substrate. (a) 2θ ranges from 12° to 75°; (b) A magnified interval near β-FeSe (001) peak from 14° to 18°. The peak position of #UFM0 and #UFM3 are marked by dashed lines, indicating a clear peak shift of FeSe (001) toward lower 2θ angle in Mg-coated #UFM3.

Figure 2

The results of resistivity behaviours of all samples in this work. (a) The temperature dependence of normalized resistivity ρ/ρ^300K up to 20 K. The y-axis is in logarithmic scale. Inset: temperature range up to 300 K. The y-axis is in linear scale; (b) ρ-T measurements for #UFM3 under external fields up to 6 T (parallel to c-axis); (c) Plots of H_c2 as a function of T_c^mid for #UFM3 and #UFM5. Inset: the linear extrapolations to T = 0 K; (d) The evolution of T_c^onset, T_c^zero, and ΔT_c with different amount of Mg-coating. The left y-axis refers to the T_c^onset and T_c^zero, and the right y-axis stands for the ΔT_c.

Figure 3

Hall coefficient R_H as a function of temperature for #UFM0, #UFM1, #UFM3 and #UFM5. (a) R_H-T plots in a temperature range from 20 K to 300 K; (b) A magnified area from 90 K to 220 K, showing the phenomenon of sign reversal in #UFM3 and #UFM5.

Figure 4

Scanning transmission electron microscopy (STEM) analyses for #UFM3 sample focusing on the cross-sectional region covering the entire FeSe layer. (a) Dark-field image. The inset provides a fast Fourier transform (FFT) pattern for FeSe core layers (zone axis $$\left[0\overline{1}0\right]$$); (b) A linear profile based on the contrast fluctuation of the arrow indicated in (a). The left/right side refers to the region of Mg-coating/CaF₂ substrate. The width values represent the c-axis lattice parameters of each FeSe unit layer.

Figure 5

The STEM-related characterizations for #UFM3 from a cross-sectional view. (a) EDS linear-scanning showing the distribution of Mg, O, Fe, Se and Ca elements; (b) The EELS contour image illustrating the evolution of Mg-K and Se-K edges. The region in which Mg and Se coexist is highlighted between dashed lines; (c) A bright-field STEM image captured from the zone axis of $$\left[\overline{1}01\right]$$; (d) The FFT pattern of the polycrystalline region in the square in (c).