The preface: Nickelates—a new playground for high-T_c superconductivity Open Access

The pursuit of high-temperature superconductors, characterized by a superconducting critical temperature (T_c) exceeding the McMillan limit, remains a central frontier in condensed matter physics. The experimental journey into nickelate superconductivity began in 2019 with the observation of superconductivity below 15 K in doped infinite-layer nickelate Nd_0.8Sr_0.2NiO₂ [1]. Subsequent discovery of superconductivity near 80 K in the bilayer Ruddlesden–Popper (RP) phase nickelate La₃Ni₂O₇ under pressure spurred extensive research into the RP phases of nickelates [2]. A recent breakthrough, the realization of superconductivity above 40 K at ambient pressure in thin-film bilayer RP nickelates, further solidifies nickelates as a distinct high-T_c family alongside cuprates and iron-based superconductors [3,4]. The NSR Special Topic reviews the status of nickelate superconductors and presents experimental and theoretical studies probing the superconducting mechanism in both bulk and thin-film samples.

The review paper by Wang et al. summarizes progress since the theoretical proposal of superconductivity in doped LaNiO₂ last century [5]. Nickelates exhibit diverse structures and valence states. While hole-doped infinite-layer ‘112’ nickelates share similarities with cuprates, possessing a 3d⁹ configuration and a hole-doping phase diagram, key differences emerge: their parent state is a Mott insulator rather than a charge transfer insulator, and their electronic structure involves significant contributions from rare-earth 5d orbitals. The review details progress and challenges in RP nickelates, including high-pressure synthesis requirements, sample quality issues (apical oxygen vacancies, stacking faults) and widely studied spin/charge density waves at ambient pressure. Discoveries of superconductivity in other pressurized RP phases (e.g. La₄Ni₃O₁₀ and La₅Ni₃O₁₁) and ambient-pressure thin-film La₃Ni₂O₇ samples offer new pathways to elucidate the superconducting mechanism.

Direct diamagnetic susceptibility measurements under high pressure are challenging. In a research article, Li et al. report the first measurements of the Meissner effect in La₃Ni₂O₇ using a superconducting quantum interference device (SQUID), extending the superconducting phase diagram up to 104 GPa [6]. They find superconductivity emerges coincidently with the straightening of the Ni–O–Ni bond angle along the c-axis. Wen et al. present a study directly imaging the Meissner effect in bilayer RP nickelates using nitrogen-vacancy centers on diamond anvil culets [7]. Combined with simultaneous Raman measurements, their work highlights the impact of inhomogeneous pressure on structural transitions. Wang et al. propose a theoretical model for bilayer RP nickelates as a self-doped molecular Mott insulator, where superconductivity may arise from two nearly degenerate antisymmetric molecular orbitals of d_x²-y² and d_z² [8].

Leveraging the gigantic oxidative atomically layer-by-layer epitaxy method, Li et al. grew high-quality superconducting La_2.85Pr_0.15Ni₂O₇ thin films with T_c above 40 K on SrLaAlO₄ substrates, enabling band structure measurements in the superconducting state [9]. Their angle-resolved photoemission spectroscopy (ARPES) study reveals contributions from both Ni d_x²-y² and d_z² orbitals at the Fermi level. Comparison with ambient-pressure bulk samples indicates hole-doping in films, likely due to interfacial Sr diffusion. Complementing this, Yue et al. employ density functional theory plus cluster dynamical mean-field theory, incorporating the on-site Coulomb repulsive U and experimentally measured electron filling n, to successfully reproduce the thin-film electronic band structure [10]. Their analysis suggests an s^±-wave pairing instability driven by strong spin fluctuations and underscores the importance of electronic correlations.

Collectively, these articles highlight significant progress and ongoing challenges in nickelate superconductivity, deepening our understanding of high-T_c mechanisms. We hope this Special Topic inspires further innovative research in this rapidly evolving field.

ACKNOWLEDGEMENTS

The authors acknowledge the support from the 397th Shuangqing Forum and the First Shuangqing-NanAo Youth Forum.

Conflict of interest statement. None declared.

REFERENCES

Author notes