Experimental research progress of charge order of nickelate based superconductors

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 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 wavevector of Q// approximate to (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 NiO2 plane,which carries unconventional superconductivity, but rather originates from the ordered excess apical oxygen in the partially reduced impurity phases. On the other hand, the overdoped lowvalence nickelate La4Ni3O8 shows well-defined intertwined charge and magnetic order, with an in-plane important role in the multi-orbital contribution of charge order formation in this material, which is significantly different from the cuprates with oxygen orbitals dominating the charge modulation. Although the spin order in La3Ni2O7 has been well established, there is still controversy over its spin structure and the existence of 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.