Daily Overview: Today’s highlights focus on advancing the understanding of the electronic structure of hybrid Ruddlesden-Popper nickelates. [1] By measuring the complete phase diagram of Nd({1-x})Sr(x)NiO(2) thin films via ellipsometry and employing a two-band Drude model, strong correlations and multiband superconducting characteristics were revealed: the spectral weights of the electron and hole bands undergo reconstruction with doping, and at optimal doping, both bands contribute to the superconducting condensation, directly confirming the multiband superconducting nature. [2] Using a neural quantum state method to simulate the mixed-dimensional (t\parallel)-(J\parallel)-(J\perp) model describing high-pressure bilayer nickelate La(_3)Ni(_2)O(_7), direct numerical evidence of ground-state superconductivity was obtained on an 8×8×2 lattice. A crossover from BEC-type pairing under strong interlayer coupling to BCS-type pairing under weak coupling was observed, along with a first-order phase transition from interlayer s-wave to intralayer d-wave pairing symmetry. These two works deepen the understanding of electronic correlations, multiband pairing, and pairing mechanisms in nickel-based superconductors from the perspectives of experimental spectroscopy and theoretical simulation, respectively. arXiv submission processing window: 2026-02-10 09:18 to 2026-02-10 18:55 UTC.

1. Optical conductivity signatures of strong correlations and multiband superconductivity in infinite-layer nickelates

  • Relevance Score: 5.2315
  • Authors: Woo Jin Kim, Kyuho Lee, Eun Kyo Ko, Jaeseok Son, Yonghun Lee, Yijun Yu, Soon Jae Moon, Tae Won Noh, Harold Y. Hwang
  • Affiliations: Hanyang University, Institute for Basic Science, SLAC National Accelerator Laboratory, Stanford University, Seoul National University, Pusan National University, Fudan University
  • Link: http://arxiv.org/abs/2602.09567v1

Summary: By measuring the optical conductivity of Nd1-xSrxNiO2 thin films across the entire phase diagram (0.025 < x < 0.30) using ellipsometric spectroscopy and analyzing the data with a two-band Drude model, the characteristics of strong correlations and multiband superconductivity are revealed. This model decomposes the intraband response into two contributions: a narrow Drude term attributed to the electron band, and a broad Drude term corresponding to the hole band with strong correlations. As Sr doping increases, the spectral weight of the hole band increases while that of the electron band decreases, indicating a multiband electronic structure and doping-dependent Fermi surface reconstruction. Furthermore, both doping- and temperature-dependent optical spectra show significant spectral weight transfer from high to low energies, a hallmark of strong electronic correlations. In the superconducting state at optimal doping (x = 0.15), both the electron and hole bands contribute to the superconducting condensate, confirming the multiband nature of superconductivity.

2. Simulating superconductivity in mixed-dimensional $t_\parallel$-${J}_\parallel$-${J}_\perp$ bilayers with neural quantum states

Summary: This study employs a neural quantum state approach, specifically the Gutzwiller-projected hidden fermion Pfaffian state, to simulate the ground-state properties of a mixed-dimensional bilayer t_∥-J_∥-J_⊥ model—an effective low-energy model for pressurized bilayer nickelate La₃Ni₂O₇—on lattices up to 8×8×2. Numerical results reveal superconductivity over a wide range of doping concentrations and coupling strengths. By analyzing pairing behavior, the study identifies a crossover from tightly bound BEC-type interlayer pairs under strong interlayer exchange coupling to spatially extended BCS-type pairs under weak interlayer exchange. Furthermore, tuning the intralayer exchange coupling induces an abrupt change in pairing symmetry, from interlayer s-wave to intralayer d-wave pairing, characterized as a first-order phase transition. The computational accuracy is verified through comparison with matrix product state simulations on coupled ladders. This work represents the first application of neural quantum states to fermionic multi-orbital systems and, with high-precision numerical methods, provides direct evidence for superconductivity in two-dimensional bilayer systems, offering significant insights into the pairing mechanisms of bilayer nickelate superconductors and cold-atom quantum simulation platforms.