Experimental Progress in Charge Order of Nickelate Superconductors

Abstract

Ever since the discovery, nickelate superconductors have attracted great attention, declaring a “nickel age” of superconductivity. Currently, there are two categories, low-valence nickelate superconductors RE_<i>n</i>+1Ni_<i>n</i>O_2<i>n</i>+2 (RE: rare earth; <i>n</i>: number of adjacent NiO₂ layers) and high-pressure nickelate superconductors La₃Ni₂O₇ and La₄Ni₃O₁₀. Charge order plays a crucial role in the study of strongly correlated systems, especially the cuprate superconductors, in which potential correlations between charge order and superconductivity have been indicated. Thus, great efforts have been made in exploring the charge order in nickelate superconductors. In the infinite-layer nickelate RENiO₂, evidence of charge order with in-plane wavevector of <b>Q</b> _//≈ (1/3, 0) has been found in the undoped and underdoped regime but not in the superconducting samples. However, subsequent studies point out that this is not real charge order intrinsic to the NiO₂ planes that host the unconventional superconductivity but rather originating from ordered excess apical oxygens in the partially reduced impurity phase. In another aspect, the overdoped low-valence nickelate La₄Ni₃O₈ shows well-defined intertwined charge and magnetic order with in-plane wavevector of <b>Q</b> _//= (1/3, 1/3). Resonant x-ray scattering study found multi-orbital contributions to the charge order formation in this material with nickel orbitals play the most important role, which is quite different from cuprates where the oxygen orbitals dominate the charge order formation. While spin order has been well established in La₃Ni₂O₇, its spin structure and whether there is coexisting charge order remains controversial. In La₄Ni₃O₁₀, intertwined charge and spin density waves have been reported, the origin and characteristics of which remains unknown. With the study of the nickelate superconductors just initiated and so many questions unanswered, the exploration of charge order in nickelate superconductors will remain the center of superconductor study.