New sample architectures with Fe-based superconductors based on layer transfer of epitaxial films

铁基超导体具有丰富的电子相，可以通过掺杂、压力或应变进行调节。通常，单晶用于详细的研究，由于这些材料中晶界的有害影响。在过去几年中，特别是对于FeSe和BaFe2As2基材料，生长了高质量的外延薄膜，其显示出与单晶材料相当的质量。然而，这些超导层在刚性衬底上的夹持限制了这种薄膜在更高级研究中的应用。因此，该项目的主要目的是通过实现薄的独立层，为铁基超导体的研究开辟新的途径，这些独立层可以转移到其他模板上。这种方法的核心工具是牺牲层的应用，到目前为止，牺牲层主要用于钙钛矿基氧化物架构。主要的挑战是优化这些牺牲层架构的生长，释放和转移的无氧铁基超导薄膜。一个关键的方面将是减少应变，以避免在去除牺牲层以及实现清洁界面期间出现大量裂纹。除了这些材料的牺牲层结构上的生长，生长的外延膜的层转移需要详细研究，其中包括适当的蚀刻程序，以避免超导性能本身的任何恶化，以及适当的转移策略，以实现新的模板上的光滑和无裂纹的膜的发展。在该项目中，针对释放的铁基超导层的不同应用。首先，将层转移到柔性聚合物模板上，通过拉伸或弯曲这种模板来研究应变对超导转变的影响。其次，薄膜将被转移到薄膜上，这允许使用透射电子显微镜内的专用低温载物台来研究这些材料。最后，这些层将用于实现新的层结构，例如通过将层彼此扭转或通过将超导体与其他功能材料如磁体或铁电体组合以创建多功能异质结构。在所有情况下，主要针对选定应用与经验丰富的合作伙伴合作进行原理验证研究。我们希望该项目为铁基超导体的基础研究创造新的机会，这可能有助于更好地理解这些材料的超导机制。与此同时，所开发的方法将使新的设备架构的铁基超导体在功能异质结构的潜在应用的实现。

Fe based superconductors show a rich variety of electronic phases, which can be tuned by doping, pressure or strain. Typically, single crystals are used for detailed studies due to the detrimental effect of the grain boundaries in these materials. Within the last years, high-quality epitaxial thin films were grown in particular for FeSe and BaFe2As2 based materials showing a comparable quality as single crystalline materials. However, the clamping of these superconducting layers on the rigid substrates limits the application of such films for more advanced studies. Therefore, the major aim of this project is to open new pathways for the study of Fe-based superconductors by the realization of thin freestanding layers, which can be transferred to other templates. The central tool for this approach is the application of sacrificial layers, which are mainly used so far for perovskite-based oxide architectures. The main challenge is to optimize these sacrificial layer architectures for the growth, release and transfer of oxygen-free Fe based superconductor films. A key aspect will be the reduction of strain to avoid a high number of cracks during the removal of the sacrificial layer as well as the realization of clean interfaces. Besides the growth of these materials on sacrificial layer architectures, the layer transfer of the grown epitaxial films needs to be studied in detail, which includes the development of suitable etching procedures to avoid any deterioration of the superconducting properties itself as well as of an appropriate transfer strategy to achieve smooth and crack-free films on the new template. Within the project, different applications of the released Fe-based superconducting layers are targeted. At first, layers will be transferred to flexible polymer template to study the influence of strain on the superconducting transition by stretching or bending such templates. Secondly, films will be transferred to thin membranes, which allow to study these materials using a dedicated low-temperature stage inside a transmission electron microscope. Finally, such layers will be used to realize new layer architecture as for example by twisting the layers against each other or by combining the superconductor with other functional materials as magnets or ferroelectrics to create multifunctional heterostructures. In all cases, a proof-of-principle study is mainly targeted in collaboration with experienced partners for the selected applications. We expect that the proposed project creates new opportunities for basic studies on Fe-based superconductors, which might help to understand the mechanism of superconductivity in these materials better. At the same time, the developed approaches will enable the realization of new device architectures for potential applications of Fe-based superconductors in functional heterostructures.