Paper

In 2023, the bilayer nickel oxide La₃Ni₂O₇ was discovered to exhibit superconductivity at a pressure of approximately 14 GPa with a critical temperature near 80 K, featuring a structure similar to that of high-temperature copper oxides; subsequently, superconductivity was also found in the trilayer compound La₄Ni₃O₁₀. These two compounds belong to the Ruddlesden–Popper phase, which consists of alternating stacks of NiO₂ square-lattice layers and LaO rock-salt layers. Current research is mainly pursued along three directions: expanding the chemical diversity of the compounds, raising the superconducting transition temperature through elemental substitution, and elucidating the pairing mechanism of superconductivity. However, key experiments must be conducted under high pressure, which poses difficulties for mechanistic studies; therefore, developing nickel oxides that exhibit superconductivity at lower pressures or even ambient pressure is of great significance. This review summarizes the existing knowledge of these systems, highlights the relatively mature methods for sample synthesis and characterization, and briefly outlines their electronic properties, aiming to provide a foundation for future material exploration and physical understanding of the underlying mechanisms.

This study systematically constructs the superconducting phase diagram of multilayer square-planar nickelates Nd_{n+1}Ni_nO_{2n+2} (n = 4–8), revealing that compounds with n = 4 to 7 exhibit signs of superconductivity, with a maximum onset critical temperature of 12.9 K (n = 6), while n = 8 shows only weak superconducting correlations. As the layer number n decreases, the superconducting anisotropy undergoes a reversal due to the effect of 4f electrons at the neodymium sites—the electronic structure approaches that of cuprates, and magnetic fluctuations persist in both the superconducting region and the overdoped nonsuperconducting region. Notably, this superconducting region overlaps with that of chemically doped infinite-layer nickelates, highlighting commonalities and differences among different structural realizations in square-planar nickelates. This work establishes a general template for synthesizing novel nickel-based superconductors through atomic-precision layered design.

This study investigates the superconductivity and magnetism of bilayer nickelates from an itinerant perspective. Based on tight-binding fitting of angle-resolved photoemission spectroscopy data from compressively strained films, the authors introduced standard on-site repulsive interactions (including intra-orbital U, inter-orbital U′, Hund’s coupling JH, and pair-hopping JP) and renormalized these bare interactions through the random phase approximation (RPA) by considering particle-hole fluctuations, thereby obtaining an effective pairing interaction. The results show that in the strong Hund’s coupling regime, s-wave superconductivity and (π/2, π/2) spin density wave (SDW) order are the dominant ground states, while under weak Hund’s coupling, d-wave pairing and (π, π) SDW become the leading ground states. These findings are qualitatively consistent with previous density matrix renormalization group (DMRG) studies, underscoring the critical role of Hund’s coupling in determining the superconducting pairing symmetry and magnetic type of the system.

The discovery of superconductivity under high pressure in Ruddlesden–Popper phases of bulk nickelates has sparked great interest in stabilizing ambient-pressure superconductivity in the thin-film form using epitaxial strain. Recently, signs of superconductivity have been observed in compressively strained bilayer nickelate thin films with an onset temperature exceeding 40 K, although with broad, two-step-like transitions. Here we report the intrinsic superconductivity and normal-state transport properties in compressively strained La2PrNi2O7 thin films, achieved through a combination of isovalent Pr substitution, growth optimization and precision ozone annealing. The superconducting onset occurs above 48 K, with zero resistance reached above 30 K, and the critical current density at 1.4 K is 100-fold larger than previous reports. The normal-state resistivity exhibits quadratic temperature dependence indicative of Fermi liquid behaviour, and other phenomenological similarities to transport in overdoped cuprates suggest parallels in their emergent properties.

The discovery of unconventional superconductivity in (La,Ba)2CuO4 (ref. 1) has motivated the study of compounds with similar crystal and electronic structure, with the aim of finding additional superconductors and understanding the origins of copper oxide superconductivity. Isostructural examples include bulk superconducting Sr2RuO4 (ref. 2) and surface-electron-doped Sr2IrO4, which exhibits spectroscopic signatures consistent with a superconducting gap3,4, although a zero-resistance state has not yet been observed. This approach has also led to the theoretical investigation of nickelates5,6, as well as thin-film heterostructures designed to host superconductivity. One such structure is the LaAlO3/LaNiO3 superlattice7–9, which has been recently proposed for the creation of an artificially layered nickelate heterostructure with a singly occupied $${d}_{{x}^{2}-{y}^{2}}$$band. The absence of superconductivity observed in previous related experiments has been attributed, at least in part, to incomplete polarization of the eg orbitals10. Here we report the observation of superconductivity in an infinite-layer nickelate that is isostructural to infinite-layer copper oxides11–13. Using soft-chemistry topotactic reduction14–20, NdNiO2 and Nd0.8Sr0.2NiO2 single-crystal thin films are synthesized by reducing the perovskite precursor phase. Whereas NdNiO2 exhibits a resistive upturn at low temperature, measurements of the resistivity, critical current density and magnetic-field response of Nd0.8Sr0.2NiO2 indicate a superconducting transition temperature of about 9 to 15 kelvin. Because this compound is a member of a series of reduced layered nickelate crystal structures21–23, these results suggest the possibility of a family of nickelate superconductors analogous to copper oxides24 and pnictides25.

The high-temperature superconductivity in bilayer La₃Ni₂O₇ originates from its unique two-orbital bilayer electronic structure, where the 3d_z² orbital is nearly half-filled and localized, generating strong interlayer antiferromagnetic exchange via the inner apical oxygen 2p_z orbital, while the 3d_x²-y² orbital is approximately quarter-filled and highly itinerant. Under strong coupling, Hund’s rule coupling aligns the spins of the two orbitals on the same nickel site, effectively transferring the interlayer antiferromagnetic exchange to the itinerant 3d_x²-y² orbital, forming an effective coupling J⊥. This mechanism can be simplified into a strong-coupling bilayer t-J-J⊥ model for the 3d_x²-y² band, where J⊥ drives electrons to form interlayer Cooper pairs, realizing extended s-wave pairing superconductivity with high critical temperature. Meanwhile, the strongly localized 3d_z² electrons tend to form interlayer ladder singlets; due to the lack of phase coherence, these singlets do not directly participate in the superconducting condensation but instead give rise to a pseudogap phase. This review systematically elaborates on this strong-coupling Hund’s rule assisted pairing theory, providing a unified framework for understanding the mechanism of high-temperature superconductivity in this system.

This study employs sign-problem-free quantum Monte Carlo simulations to systematically investigate a bilayer fermionic model with strong interlayer antiferromagnetic exchange coupling and local Hubbard repulsion, which serves as a prototype for realizing the symmetric mass generation (SMG) insulator. The numerically exact results unambiguously demonstrate that robust superconducting pairing emerges upon doping the SMG insulator phase, and the Hubbard repulsion significantly enhances the superconducting order parameter. Given that this model may capture key features of the high-temperature superconductor La₃Ni₂O₇ under high pressure, this work establishes a new paradigm for achieving superconductivity starting from a doped SMG parent state, providing important theoretical guidance for future experimental exploration.

The discovery of 80 K superconductivity in pressurized bilayer Ruddlesden-Popper (RP) nickelate La$_3$Ni$_2$O$_7$ has established a new high-temperature superconductor family. The quest to understand the governing principles of RP nickelate superconductivity has become a central focus in condensed matter physics. Here, we report a critical advance by synthesizing and investigating a distinct structural polymorph of the same compound: the monolayer-trilayer (1313) hybrid phase of La$_3$Ni$_2$O$_7$. Under high pressure, synchrotron X-ray diffraction and Raman spectroscopy reveal a structural transition from the orthorhombic $Cmmm$ to the tetragonal $P4/mmm$ space group at 13~GPa. Above 19 GPa, the phase exhibits a clear superconducting transition, confirmed by a zero-resistance state, albeit at a significantly reduced temperature of 3.6 K. The stark contrast with the 80 K transition in the bilayer phase provides a uniquely clean experimental comparison. Our results demonstrate that the superconducting transition temperature is directly governed by the nature of the interlayer coupling, and the bilayer NiO$_6$ block as the essential structural motif for achieving high-$T_\text{c}$ superconductivity in the RP nickelates.

Superconductivity in pressurized trilayer La₄Ni₃O₁₀−δ single crystals

In recent years, significant breakthroughs have been achieved in the study of Ruddlesden-Popper (RP) nickel oxide superconductors. This article systematically reviews the experimental and theoretical progress in this field, with a particular focus on thin-film systems. Key findings include the emergence of superconductivity in bilayer La₃Ni₂O₇ (T_c ~ 80 K) and trilayer La₄Ni₃O₁₀ under high pressure, and, critically, the realization of ambient-pressure superconductivity in ultra-thin films of La₃Ni₂O₇ grown on substrates providing compressive strain—a breakthrough that overcomes the high-pressure limitation and enables the use of experimental techniques previously inaccessible in the superconducting state, such as angle-resolved photoemission spectroscopy (ARPES). On the theoretical side, the system requires simultaneous consideration of both the Ni e_g and a_{1g} orbitals, as well as the strong interlayer coupling within bilayers that gives rise to a “dimer” picture, and exhibits strange metal behavior and strong correlation features reminiscent of cuprates. By comparing the similarities and differences among various RP nickel oxides, this article offers a new perspective on understanding the mechanism of high-temperature superconductivity in correlated electron systems and outlines future research directions.