Abstract
The recent discovery of superconductivity in infinite-layer nickelate films has aroused great interest since it provides a new platform to explore the mechanism of high-temperature superconductivity. However, superconductivity only appears in the thin film form and synthesizing superconducting nickelate films is extremely challenging, limiting the in-depth studies on this compound. Here, we explore the critical parameters in the growth of high-quality nickelate films using molecular beam epitaxy. We found that stoichiometry is crucial in optimizing the crystalline structure and realizing superconductivity in nickelate films. In precursor NdNiO3 films, optimal stoichiometry of cations yields the most compact lattice while off-stoichiometry of cations causes obvious lattice expansion, influencing the subsequent topotactic reduction and the emergence of superconductivity in infinite-layer nickelates. Surprisingly, in-situ reflection high energy electron diffraction indicates that some impurity phases always appear once Sr ions are doped into NdNiO3 although the X-ray diffraction data are of high quality. While these impurity phases do not seem to suppress the superconductivity, their impacts on the electronic and magnetic structure deserve further studies. Our work demonstrates and highlights the significance of cation stoichiometry in the superconducting nickelate family.
Highlights
Over the past decades, there have been a great number of investigations on the superconductivity in nickelates, as they are natural analogs of high-Tc cuprates [1,2,3,4,5,6,7,8]
Our results demonstrate the significance of stoichiometry, which could impede the complete transformation of perovskite phase to infinite-layer phase and hinder the superconductivity when the deviation is more than 10%
Optimization of the quality of nickelate films was investigated in this work using molecular beam epitaxy (MBE)
Summary
There have been a great number of investigations on the superconductivity in nickelates, as they are natural analogs of high-Tc cuprates [1,2,3,4,5,6,7,8]. Superconductivity was eventually found in the hole-doped infinite-layer nickelates [9], which have a layered structure and 3d9−x electronic configuration similar to those of cuprate superconductors. This significant discovery provides a new platform to explore the mechanism of high-temperature superconductivity and triggers intense research interests [10,11,12,13,14,15,16]. Superconductivity has only been observed in nickelate thin films, while bulk samples show an insulating behavior [27, 28].
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