Daily Overview: Today’s highlight focuses on revealing the intrinsic electronic structure and pairing mechanism of nickelate superconductors from multiple dimensions. In superconducting bilayer nickel oxide thin films, study [1] employs resonant X-ray scattering and spectroscopy to clarify the spatial separation between superconductivity and spin density wave order, and proposes a novel interlayer five-spin polaron state in which the Ni(d⁸)-O(L)-Ni(d⁸) configuration locks out-of-plane orbital charge and spin, leaving the in-plane orbitals near half-filling, thereby elucidating the critical role of oxygen stoichiometry in tuning interlayer coupling and the electronic ground state. For the trilayer nickel oxide La₄Ni₃O₁₀, theoretical calculations in study [2] demonstrate that pressure can induce additional hole transfer from apical to planar oxygens, forming freely propagating in-plane three-spin polaron quasiparticles, thus providing a natural microscopic mechanism for the emergence of superconductivity in the high-pressure phase. Furthermore, study [3] uses ultrafast terahertz spectroscopy in infinite-layer samarium nickel oxide thin films to observe a linear temperature response of the superfluid density, extract a clean d-wave pairing gap, and confirm that the system resides in the weak-coupling clean limit, directly demonstrating a profound similarity in pairing symmetry between nickel-based superconductors and cuprate high-temperature superconductors. arXiv submission processing window: 2026-06-12 00:00 to 2026-06-12 00:00 UTC.

1. Interlayer Five-Spin Polaron in Superconducting Bilayer Nickelates

  • Relevance Score: 5.6972
  • Authors: Jiarui Li, Christopher T. Parzyck, Eder G. Lomeli, Yidi Liu, Taehun Kim, Heemin Lee, Zengqing Zhuo, Eun Kyo Ko, Yaoju Tarn, Cheng-Tai Kuo, Ronny Sutarto, Chunjing Jia, Vivek Thampy, Jonathan Pelliciari, Wanli Yang, Brian Moritz, Yijun Yu, Jun-Sik Lee, Valentina Bisogni, Thomas P. Devereaux, Harold Y. Hwang, Wei-Sheng Lee
  • Affiliations: University of Florida, Stanford University, Lawrence Berkeley National Laboratory, Brookhaven National Laboratory, Canadian Light Source, SLAC National Accelerator Laboratory
  • Link: https://arxiv.org/abs/2605.02891
  • Paper page: Interlayer Five-Spin Polaron in Superconducting Bilayer Nickelates

Summary: This study employs resonant X-ray scattering and spectroscopy to investigate the relationship between spin density wave (SDW) order and superconductivity in superconducting bilayer nickelate thin films of La₂PrNi₂O₇. By comparing superconducting and oxygen-deficient samples, it is found that SDW exists exclusively in oxygen-depleted regions, whereas stoichiometric regions are free of SDW and exhibit superconductivity, indicating a spatially phase-separated coexistence. Ni-L₃ and O-K edge spectroscopy reveal that oxygen content along the c-axis profoundly modifies the electronic structure, particularly the interlayer coupling. Based on experimental findings and theoretical analysis, we propose the formation of ligand holes on the apical oxygen between layers, which stabilize an interlayer five-spin polaron state. In this state, the Ni(d⁸)-O(L)-Ni(d⁸) configuration locks the charge and spin of the out-of-plane 3dz² orbital, pushing the in-plane 3dx²-y² orbital toward a nearly half-filled condition that resembles the d⁹ ground state of cuprate superconductors. This discovery clarifies that oxygen stoichiometry is a crucial parameter for tuning interlayer coupling and the electronic ground state, offering a new perspective for understanding the superconducting mechanism in bilayer nickelates.

2. Pressure induced redistribution of oxygen hole states in La$_{4}$Ni$_{3}$O$_{10}$

Summary: Using density functional calculations and a multi-orbital, multi-atom cluster exact diagonalization approach, the low-energy electronic states of the trilayer nickelate La₄Ni₃O₁₀ are investigated via a minimal Ni₃O₁₄ cluster. At ambient pressure, one of the two extra holes localizes in the d_(x²-y²) orbital of the central NiO₂ layer forming a Zhang-Rice singlet, while the other mainly occupies the antibonding combination of interlayer apical oxygen p_z orbitals and hybridizes with the out-of-plane three-spin polaron composed of the trilayer d_(z²) orbitals. Under high pressure, the two extra holes concentrate separately in one outer layer and one inner layer, both forming Zhang-Rice singlets with d_(x²-y²) orbitals. The results indicate that the transfer of holes from apical oxygen to outer-layer planar oxygen generates an in-plane three-spin-polaron-type quasiparticle that acts as a mobile carrier coupled by interlayer superexchange, whereas the interlayer three-spin-polaron state may provide a pairing mediator. The low-pressure phase lacks freely propagating in-plane quasiparticles, so this mechanism naturally supports superconductivity in the high-pressure phase.

3. Evidence for Clean d-wave Superconductivity in Samarium Nickelates

Summary: Using ultrafast terahertz spectroscopy, we performed optical-pump terahertz-probe experiments on an infinite-layer samarium nickel oxide thin film with Tc = 20 K to measure the temperature-dependent photoconductivity. Under weak excitation, the photoinduced destruction of the superfluid density is proportional to the equilibrium superfluid density and decreases linearly with increasing temperature, consistent with clean-limit d-wave pairing. From this linear relationship, the superconducting gap was extracted to be 2.5 meV, yielding 2Δ/kTc ≈ 3, indicating the system is in the weak coupling regime. Furthermore, independent estimates of the ratio of the mean free path to the coherence length (l/ξ) give approximately 1.5, further confirming clean-limit behavior. These results demonstrate that nickel oxide superconductors can realize a clean superconducting state and reveal a close similarity in pairing mechanism to cuprate high-temperature superconductors.