Stoichiometry, Orbital Configuration, and Metal-to-Insulator Transition in Nd_0.8Sr_0.2NiO₃ Films

The discovery of superconductivity in the infinite-layer nickelate Nd_0.8Sr_0.2NiO₂ has motivated tremendous efforts for its significance toward the understanding of high-temperature superconductivity. However, the synthesis of infinite-layer nickelates is instable and has become a hindrance to experimental progress. Optimizing the growth of precursor Nd_0.8Sr_0.2NiO₃ by pulsed laser deposition is crucial for obtaining infinite-layer nickelates. By systematically investigating the growth of Nd_0.8Sr_0.2NiO₃ with wide range of conditions, we found that the laser fluence plays a critical role in determining the stoichiometry, lattice structure, and electronic properties. A higher Ni deficiency and larger c-axis lattice constant appeared with the lower laser fluence. At 0.6 J/cm², the Ni deficiency is as large as 25%. According to X-ray absorption spectra and X-ray linear dichroism, we further find that (i) there are no obvious changes of the Ni valence and (ii) the energy level of the d orbital gradually increases relative to the d₃ orbital with increasing Ni deficiency. What is more, the onset temperature and magnitude of the resistivity change at the metal-to-insulator transitions (MITs) also are found to decrease with increasing laser fluence during the growth.