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) in situ${\mathrm{\text{in situ}}}$ 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 (TR) and hydrogen pressure (PH), are systematically explored. It is found that the reduction window for achieving superconducting transition is quite wide, reaching nearly 80°C in TR and three orders of magnitude in PH when the reduction time is set to 30 min. And there exists an optimal PH for achieving the highest Tc if both TR 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 in situ${\mathrm{\text{in situ}}}$ reduction process, providing advantages over the ex situ${\mathrm{\text{ex situ}}}$ CaH2 method for surface‐sensitive experiments. The superconducting infinite‐layer nickelate films can be synthesized fully in vacuo${\mathrm{\text{in vacuo}}}$ using atomic hydrogen, while the atomically flat surface has been confirmed by atomic force microscopy and scanning transmission electron microscopy. The in situ${\mathrm{\text{in situ}}}$ reduction method paves the way for surface‐sensitive electronic structural characterizations, such as angle‐resolved photoemission spectroscopy and scanning tunneling microscopy/spectroscopy.