Unveiling a Novel Insulator-to-Metal Transition in La₂NiO_4+δ: Challenging High-Temperature Superconductivity Claimed for Single-Layer Lanthanum Nickelates

Abstract La₂NiO₄ has a similar structure to La₂CuO₄ and was proposed as a high-temperature superconductor based on magnetic-moment measurements decades ago. Nevertheless, with the exception for electrical resistance drop behavior of about 4 orders of magnitude that is claimed to originate from the superconductivity ever observed in Sr-doped La₂NiO₄, most electrical data reported to date in La₂NiO₄ system exhibit a trivial insulating ground state. Here, we definitively identify the similar electrical resistance drop behavior of more than 3 orders of magnitude in La₂NiO_4+δ. However, our extensive investigations reveal that this phenomenon is a novel insulator-to-metal transition, distinct from superconductivity. Intriguingly, compared to the weak magnetic-field effects, pressure can significantly suppress the transition and transform from the metallic to an insulating ground state, accompanied by an isostructural phase transition. Our work not only elucidates the fundamental properties of the metallic conducting ground state in La₂NiO_4+δ, but also critically challenges the notion of superconductivity in single-layer lanthanum nickelates.