Superconducting FeSe membrane synthesized by etching of water-soluble Sr₃Al₂O₆ layer

The development of the epitaxial lift-off technique using a pseudoperovskite Sr₃Al₂O₆ sacrificial layer has unleashed latent physical properties emerging in freestanding membranes, mainly composed of lattice-matched perovskite-type complex oxides. Here, we report the superconductivity in a freestanding single-crystalline FeSe membrane prepared using a SrTiO₃ capped water-soluble Sr₃Al₂O₆ sacrificial layer, which serves as an ex situ growth template. The FeSe membrane is synthesized by etching the sacrificial layer and transferred on a SiO₂/Si substrate. X-ray diffraction pattern and scanning transmission electron microscopy reveal that the FeSe membrane is fully relaxed with minimum degradation of its structural properties during the lift-off process. A superconductivity with zero resistance below 4.2 K is exhibited in the freestanding FeSe membrane, while it is not observed in a compressed thin-film form by an in-plane tensile strain. In addition, critical magnetic field and critical current density of the FeSe membrane are comparable to those of the bulk single crystal. Our demonstration of the superconducting FeSe membrane ensures a high utility of the epitaxial lift-off technique for various thin-film materials grown on SrTiO₃. This study paves the way for functional applications using ex situ thin-film growth and lift-off technique with an expanded selection from an inventory of various materials.