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

Article

关键词: HIGH-TEMPERATURE SUPERCONDUCTIVITY;    ENERGY-LOSS-SPECTROSCOPY;    X-RAY-ABSORPTION;    TRANSITION-METALS;    PHASE-DIAGRAM;    FESE FILMS;    THIN-FILM;    ELECTRON;    3D;    OCCUPANCY;   

DOI  :  10.1103/PhysRevB.99.064502

来源: SCIE

【 摘 要 】

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 (T-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 (similar to 20 nm for Fe-Se and similar to 30 nm for Fe-Se-Te) with the reason still under debate. Here, we report an enhanced T-c approximate to T 17.6 K in a 7-nm-FeTe/7-nm-FeSe bilayer heterostructure grown on CaF2 substrate. Generally, the Fe-Ch film on CaF(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-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.

【 授权许可】

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