In Situ Preparation of Superconducting Infinite-Layer Nickelate Thin Films with Atomically Flat Surface

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.