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11 linked papers

Researchers successfully synthesized a series of infinite-layer Nd₁₋ₓEuₓNiO₂ thin films with doping extended to x = 0–0.7 using pulsed laser deposition combined with calcium hydride topotactic reduction. Electrical transport measurements reveal a superconducting dome in the range 0.2 ≤ x ≤ 0.5, whose doping width is larger than that of samples fabricated by molecular beam epitaxy and comparable to that achieved by chemical solution methods. The film with x = 0.3 exhibits an optimal superconducting transition temperature of about 31 K, significantly higher than values obtained by other vacuum epitaxy techniques, indicating that pulsed laser deposition is an effective route for preparing high-quality, high-transition-temperature nickelate superconducting thin films. Magnetotransport experiments observe robust magnetic-field-enhanced and re-entrant superconductivity in both underdoped and overdoped regions, attributed to the polarization of Eu²⁺ local magnetic moments under an external field that generates an internal exchange field partially compensating the applied field; the Jaccarino-Peter effect alone cannot fully explain this phenomenon, suggesting the existence of additional mechanisms. In the low-temperature region just above the onset superconducting transition temperature, the Hall resistance exhibits a nonlinear character without noticeable magnetic hysteresis, which may arise from magnetic impurity scattering. These results highlight the critical role of magnetic rare-earth Eu²⁺ ions in imparting exotic physical properties to infinite-layer nickelates.

The ambient-pressure superconductivity in La3⁢Ni2⁢O7 thin films via compressive epitaxial strain provides a highly accessible platform for diverse characterization techniques, facilitating the studies of high-temperature superconductivity. Here, we systematically map the phase diagram and reveal the superconducting dome with an electron-hole crossover in compressively strained La3−𝑥⁢Sr𝑥⁢Ni2⁢O7−𝛿 thin films by simultaneously tuning Sr doping and oxygen content. The maximum transition temperature (𝑇𝑐) coincides with an anomalous sign change in the Hall coefficient (𝑅𝐻), reminiscent of electron-doped cuprates, which may signal a Fermi surface reconstruction. Beyond the superconducting dome, a ln⁡1/𝑇 insulating regime and a 𝑇-linear resistivity regime are also resolved, resembling behaviors observed in cuprates and infinite-layer nickelates. This work reveals a dome-shaped relationship between 𝑇𝑐 and 𝑅𝐻 and establishes a key framework for understanding unconventional superconductivity in nickelate systems.

9 linked papers

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.

7 linked papers

34 linked papers

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.