We report the observation of superconductivity in infinite-layer Ca-doped LaNiO 2 (La 1− x Ca x NiO 2 ) thin films and construct their phase diagram. Unlike the metal-insulator transition in Nd- and Pr-based nickelates, the undoped and underdoped La 1− x Ca x NiO 2 thin films are entirely insulating from 300 K down to 2 K. A superconducting dome is observed at 0.15 < x < 0.3 with weakly insulating behavior at the overdoped regime. Moreover, the sign of the Hall coefficient R H changes at low temperature for samples with a higher doping level. However, distinct from the Nd- and Pr-based nickelates, the R H -sign-change temperature remains at around 35 K as the doping increases, which begs further theoretical and experimental investigation to reveal the role of the 4f orbital to the (multi)band nature of the superconducting nickelates. Our results also emphasize a notable role of lattice correlation on the multiband structures of the infinite-layer nickelates.

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Nickel-based complex oxides have served as a playground for decades in the quest for a copper-oxide analog of the high-temperature superconductivity. They may provide clues towards understanding the mechanism and an alternative route for high-temperature superconductors. The recent discovery of superconductivity in the infinite-layer nickelate thin films has fulfilled this pursuit. However, material synthesis remains challenging, direct demonstration of perfect diamagnetism is still missing, and understanding of the role of the interface and bulk to the superconducting properties is still lacking. Here, we show high-quality Nd0.8Sr0.2NiO2 thin films with different thicknesses and demonstrate the interface and strain effects on the electrical, magnetic and optical properties. Perfect diamagnetism is achieved, confirming the occurrence of superconductivity in the films. Unlike the thick films in which the normal-state Hall-coefficient changes signs as the temperature decreases, the Hall-coefficient of films thinner than 5.5 nm remains negative, suggesting a thickness-driven band structure modification. Moreover, X-ray absorption spectroscopy reveals the Ni-O hybridization nature in doped infinite-layer nickelates, and the hybridization is enhanced as the thickness decreases. Consistent with band structure calculations on the nickelate/SrTiO3 heterostructure, the interface and strain effect induce a dominating electron-like band in the ultrathin film, thus causing the sign-change of the Hall-coefficient.

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After the reward of more than 2 decades of pursuit on the high-Tc cuprate analog with the hope to obtain a better understanding of the mechanism of high-Tc superconductivity, the discovery of superconductivity in the infinite-layer nickelate brings more mystery to the picture than expected. Tops in the list of questions are perhaps 1) absence of superconductivity in the bulk nickelate and limited thickness of the infinite-layer phase in thin film, 2) absence of superconductivity in the La-nickelate despite it being the earliest studied rare-earth nickelate, and the role of 4 f orbital in the recipe of superconductivity, 3) absence of Meissner effect and suspect of the origin of superconductivity from the interface, 4) whether nickelate hosts similar pairing symmetry to the single-band high-Tc cuprates or multiband iron-based superconductor. In this perspective article, we will discuss the following aspects: 1) stabilization of the infinite-layer phase on the SrTiO3(001) substrate and the thickness dependency of observables; 2) rare-earth dependence of the superconducting dome and phase diagram of the (La/Pr/Nd)- infinite-layer nickelate thin film; 3) experimental aspects of the measurement of Meissner effect; 4) theoretical framework and experimental study of the pairing symmetry of infinite-layer nickelate superconductor.