Unveiling a novel insulator-to-metal transition in La2NiO4+δ: Challenging hightemperature superconductivity claims in single-layer lanthanum nickelates

Abstract Decades ago, La 2 NiO 4 featuring similar structure as copper-based superconducting parent La 2 CuO 4 , was proposed to be a high-temperature superconductor based on magneticmoment measurements. Nevertheless, with the exception for an electrical resistance drop behavior of about 4 orders of magnitude that is claimed to originate from the superconductivity ever observed in Sr-doped La 2 NiO 4 , most electrical data reported to date in La 2 NiO 4 system exhibit a trivial insulating ground state. Here, we definitively identify a similar electrical resistance drop behavior of more than 3 orders in La 2 NiO 4+δ . However, our extensive investigations reveal this phenomenon to be 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 La 2 NiO 4+δ , but also critically challenges the notion of superconductivity in single-layer lanthanum nickelates.