Improvement of superconducting properties in La1−xSrxNiO2 thin films by tuning topochemical reduction temperature

The recently discovered nickelate superconductor with an infinite $\mathrm{Ni}{\mathrm{O}}_{2}$ layer is synthesized via topochemical reduction with hydride reductant, which selectively removes apical oxygens from the perovskite precursor phase. While this thermodynamic process plays a crucial role in the materialization of the superconducting nickelate, optimizing the process is challenging due to the difficulty of controlling the selective disconnection and diffusion of oxygen in the perovskite phase. Here we demonstrate a significant improvement in the superconducting properties of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Ni}{\mathrm{O}}_{2}$ by tuning the topochemical reduction temperature. By gradually increasing the reduction temperature, the ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Ni}{\mathrm{O}}_{2}$ films exhibit a transformation from the insulating state into the superconducting state, reaching a maximum onset of superconducting transition temperature ${T}_{\mathrm{c}}^{\mathrm{onset}}$ of $\ensuremath{\sim}14$ K at $x=0.20$. Moreover, the normal state metallicity is drastically enhanced despite the slight variation in the crystal structure, implying that the electrical conduction in $\mathrm{Ni}{\mathrm{O}}_{2}$ planes is likely a sensitive parameter to optimize the reduction state. The tunings of the reduced state in ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Ni}{\mathrm{O}}_{2+\ensuremath{\delta}}$ films with various Sr content $x$ also corroborate that the optimal reduction temperature for inducing superconductivity increases with increasing $x$. The systematic optimization of reduction temperature results in an expansion of superconducting dome in temperature versus $x$ phase diagram with higher ${T}_{\mathrm{c}}^{\mathrm{onset}}$ and wider $x$ region $(0.12\ensuremath{\le}x\ensuremath{\le}0.28)$ compared to the previously reported diagram. Our findings provide a comprehensive understanding of the topochemical reduction process and its role in the electronic transport properties, leading to significant implications for the synthesis of low-valent nickelate superconductors.