Decoupling of substrate and epitaxially grown Y123 thin films by Ca2Nb3O10 nanosheet templates

The growth of high-quality superconducting ReBCO thin films, notably YBa2Cu3O7 (Y123), is highly dependent on the substrate. To enable epitaxial growth, choosing a substrate that matches the lattice parameter of the orthorhombically structured Y123 is critical. The manufacturing of commercial ReBCO tape is performed on an MgO buffer layer [1], while for thin film growth of ideal model systems i.e. perovskite single crystals such as SrTiO3, are used as substrates [2]. The necessity to use these types of growth templates for both fundamental research and applications provides the opportunity for alternatives, where superconducting ReBCO can be grown in high quality on any arbitrary substrate. For this a flexible method is needed, where epitaxy in the film is completely independent of the type of substrate used. Realising this will provide new opportunities for the utilization of ReBCO materials like Y123.

Our research demonstrates, the successful epitaxial growth of Y123 thin films on amorphous substrates using pulsed laser deposition (PLD). Contrary to the growth on single-crystalline substrates, here, the growth template is provided by a layer of flexible, crystalline Ca2Nb3O10 nanosheets [3]. These nanosheets are deposited on amorphous substrates by Langmuir – Blodgett method. Nanosheets are one-unit cell thick 2D structures with sheet sizes around 200μm2. On the nanosheets, domains of epitaxial Y123 are realized. All the domains display out-of-plane direction uniformity but in-plane orientation diversity. Moreover, grain boundaries are formed between the domains acting as resistive barriers, which decrease the film’s superconducting performance [4]. Yet, by controlling the geometry of the nanosheets, the domain sizes are dramatically enlarged, and the in-plane mosaic twist is reduced. This results in the realization of Y123 thin films with a critical temperature (Tc) of zero resistance of 85K (figure 1) which is close to the standard of 90K for Y123 thin films [2].

Ongoing research entails a detailed investigation of the electronic properties of the Y123 thin films. More precisely, the characterization of the critical current (Ic) and its dependence on the nanosheets due to the still-existing grain boundaries and in-plane differences between domains reduce the critical current [4]. To fully characterize the Y123 films, we are using an ensemble of X-ray, electrical transport techniques, and imaging techniques, which include AFM and SEM. The deposition of nanosheets by the Langmuir–Blodgett method is scalable, flexible and applicable on a wide range of substrates. This provides opportunities of direct superconductor growth on large area (wafer-scale) and curved surfaces. Achieving this opens possibilities for small superconducting devices, like pattered coils.