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

Abstract

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 ($\ensuremath{\sim}20\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$ for Fe-Se and $\ensuremath{\sim}30\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$ for Fe-Se-Te) with the reason still under debate. Here, we report an enhanced ${T}_{\mathrm{c}}\ensuremath{\approx}17.6\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ 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.