Since their discovery, the infinite-layer nickelates have been regarded as an appealing system for gaining deeper insights into high-temperature superconductivity (HTSC). However, the synthesis of superconducting samples has been proven to be challenging. Here, an ultrahigh vacuum (UHV) $\text{in situ}$ reduction method is developed using atomic hydrogen as a reducing agent and is applied in the lanthanum nickelate system. The reduction parameters, including the reduction temperature (T_R) and hydrogen pressure (P_H), are systematically explored. It is found that the reduction window for achieving superconducting transition is quite wide, reaching nearly 80°C in T_R and three orders of magnitude in P_H when the reduction time is set to 30 min. And there exists an optimal P_H for achieving the highest T_c if both T_R and reduction time are fixed. More prominently, as confirmed by atomic force microscopy and scanning transmission electron microscopy, the atomically flat surface can be preserved during the $\text{in situ}$ reduction process, providing advantages over the $\text{ex situ}$ CaH₂ method for surface-sensitive experiments.

中文翻译：

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原位制备具有原子级平坦表面的超导无限层镍酸盐薄膜


自发现以来，无限层镍酸盐一直被认为是深入了解高温超导（HTSC）的一个有吸引力的系统。然而，超导样品的合成已被证明具有挑战性。这里，开发了一种使用原子氢作为还原剂的超高真空（UHV） $\text{in situ}$ 还原方法，并将其应用于镍酸镧体系。系统地探讨了还原参数，包括还原温度（T _R ）和氢气压力（P _H ）。发现实现超导转变的还原窗口相当宽，当还原时间设置为时，T _R 达到近 80°C，P _H 达到三个数量级30分钟。如果 T _R 和缩减时间都固定，则存在实现最高 T _c 的最佳 P _H 。更突出的是，原子力显微镜和扫描透射电子显微镜证实，在 $\text{in situ}$ 还原过程中可以保留原子级平坦的表面，比 $\text{ex situ}$ CaH ₂