Freestanding perovskite and infinite-layer nickelate membranes

Following the discovery of superconductivity in hole-doped NdNiO\(_2\) 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)\(_3\)Al\(_2\)O\(_6\) sacrificial layers to fabricate freestanding perovskite nickelate membranes and explore topotactic reduction without the SrTiO\(_3\) substrate as a template. NdNiO\(_3\)-based heterostructure membranes transferred from a LaAlO\(_3\) substrate exhibit better metallic behavior and higher hysteresis than those transferred from SrTiO\(_3\), owing to the different strain states that the NdNiO\(_3\) layer experience when grown on these two substrates. Despite the expected X-ray diffraction shifts, membranes reduced with CaH\(_2\) 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.