Experimental research progress of charge order of nickelate based superconductors

Abstract

Ever since the discovery, nickelate superconductors have attracted great attention, declaring a “nickel age” of superconductivity. Currently, there are two types of nickelate superconductors: low-valence nickelate superconductors RE<sub><i>n</i>+1</sub>Ni<sub><i>n</i></sub>O<sub>2<i>n</i>+2</sub> (RE, rare earth; <i>n</i>, number of adjacent NiO<sub>2</sub> layers) and high-pressure nickelate superconductors La<sub>3</sub>Ni<sub>2</sub>O<sub>7</sub> and La<sub>4</sub>Ni<sub>3</sub>O<sub>10</sub>. Charge order plays a crucial role in studying the strongly correlated systems, especially the cuprate superconductors, in which potential correlation between charge order and superconductivity has been indicated. Thus, great efforts have been made to explore the charge order in nickelate superconductors. In the infinite-layer nickelate RENiO<sub>2</sub>, the evidence of charge order with in-plane wavevector of <i> <b>Q</b> </i><sub>//</sub> ≈ (1/3, 0) has been found in the undoped and underdoped regime but not in the superconducting samples. However, subsequent studies have indicated that this is not the true charge order inherent in the NiO<sub>2</sub> plane,which carries unconventional superconductivity, but rather originates from the ordered excess top oxygen in the partially reduced impurity phases. On the other hand, the overdoped low-valence nickelate La<sub>4</sub>Ni<sub>3</sub>O<sub>8</sub> shows well-defined intertwined charge and magnetic order, with an in-plane wavevector of <i> <b>Q</b> </i><sub>//</sub> = (1/3, 1/3). X-ray resonant scattering study has found that nickel orbitals play the most important role in the multi-orbital contribution of charge order formation in this material, which is significantly different from the dominant charge order formation of oxygen orbitals in cuprates. Although the spin order in La<sub>3</sub>Ni<sub>2</sub>O<sub>7</sub> has been well established, there is still controversy over its spin structure and the existence of coexisting charge order. In La<sub>4</sub>Ni<sub>3</sub>O<sub>10</sub>, intertwined charge and spin density waves have been reported, the origin and characteristics of which remain unknown. Owing to the research on the nickelate superconductors just starting, many questions have not yet been answered, and the exploration of charge order in nickelate superconductors will still be the center of superconductor research.