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

Abstract

Unlike monolayer Fe-Chalcogenide (Fe-Ch)/SrTiO3 (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 CaF2 generally show the highest superconducting transition temperature (Tc) 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 (∼20nm for Fe-Se and ∼30nm for Fe-Se-Te) with the reason still under debate. Here, we report an enhanced Tc≈17.6K in a 7-nm-FeTe/7-nm-FeSe bilayer heterostructure grown on CaF2 substrate. Generally, the Fe-Ch film on CaF2 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-L3/L2 ratio near the FeTe/FeSe interface. In detail, a several-unit-cell-thick Fe(Se,Te) diffusion layer shows a higher Fe-L3/L2 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 Tc 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.

Source-reported materials — not catalogue approval

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