$\mathit{\text{In}}\mathit{\text{situ}}$ 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 proved to be challenging. Here, we develop an ultrahigh vacuum (UHV) $\mathit{\text{in}}\mathit{\text{situ}}$ reduction method using atomic hydrogen as reducing agent and apply it in lanthanum nickelate system. The reduction parameters, including the reduction temperature (${\mathit{\text{T}}}_{\mathrm{R}}$) and hydrogen pressure (${\mathit{\text{P}}}_{\mathrm{H}}$), are systematically explored. We found that the reduction window for achieving superconducting transition is quite wide, reaching nearly 80${}^{\circ }$C in ${\mathit{\text{T}}}_{\mathrm{R}}$ and 3 orders of magnitude in ${\mathit{\text{P}}}_{\mathrm{H}}$ when the reduction time is set to 30 mins. And there exists an optimal ${\mathit{\text{P}}}_{\mathrm{H}}$ for achieving the highest ${\mathit{\text{T}}}_{\mathrm{c}}$ if both ${\mathit{\text{T}}}_{\mathrm{R}}$ 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 $\mathit{\text{in}}\mathit{\text{situ}}$ reduction process, providing advantages over the $\mathit{\text{ex}}\mathit{\text{situ}}$ CaH${}_2$ method for surface-sensitive experiments.