Freestanding perovskite and infinite-layer nickelate membranes

Following the discovery of superconductivity in hole-doped NdNiO₂ infinite-layer thin films, extensive research has been conducted particularly to compare these materials with cuprates. Superconductivity has also been observed in nickelate thin films with other rare-earth elements like Pr and La, but not in their bulk forms, suggesting a critical role for substrate-induced strain/interface or dimensionality effects. In this study, we use water-soluble (Ca,Sr)₃Al₂O₆ sacrificial layers to fabricate freestanding perovskite nickelate membranes and explore topotactic reduction without the SrTiO₃ substrate as a template. NdNiO₃-based heterostructure membranes transferred from a LaAlO₃ substrate exhibit better metallic behavior and higher hysteresis than those transferred from SrTiO₃, owing to the different strain states that the NdNiO₃ layer experience when grown on these two substrates. Despite the expected X-ray diffraction shifts, membranes reduced with CaH₂ display insulating characteristics, similar to bulk infinite-layer nickelates. Our findings strongly indicate that a template is necessary to stabilize a coherent and robust infinite-layer phase with optimal transport properties.