Daily Overview: Today’s highlights focus on an in-depth analysis of the microscopic mechanisms and low-energy physics of superconductivity in bilayer nickelates. One study constructs a pairing theory based on interlayer triplon excitations, successfully reproducing the double-peak structure and opposite gap signs observed in experimental tunneling spectra. It identifies interband s± pairing as the dominant characteristic and reveals that the α band, despite its smaller density of states, hosts the larger superconducting gap. Another work employs density functional theory combined with cluster dynamical mean-field theory to show that the low-energy states are formed by the hybridization of a dynamic singlet from interlayer 3z²−r² orbitals with itinerant x²−y² orbitals. It further elucidates the qualitatively distinct tuning of orbital selectivity and itinerancy by hydrostatic pressure and in-plane strain, providing a unified framework for understanding the experimental discrepancies between bulk and strained thin films. arXiv submission processing window: 2026-06-08 00:00 to 2026-06-08 00:00 UTC.

1. Triplon-mediated pairing and the superconducting gap structure in bilayer nickelates

Summary: This study constructs a microscopic model for the bilayer nickelate superconductor, incorporating a conduction band of (d_{x^2-y^2}) symmetry and localized (d_{3z^2-r^2}) spins. Strong interlayer coupling drives the local moments into a singlet ground state, whose virtual singlet–triplet excitations (triplons) serve as the pairing glue. By taking into account orbital hybridization, Hund’s coupling, and nonlocal Kondo exchange, the effective interaction between band electrons and localized spins is derived, yielding a BCS Hamiltonian. The theory leads to interband (s\pm) pairing, with order parameters on the (\alpha) and (\beta) bands exhibiting opposite signs and different magnitudes—remarkably, the (\alpha) band, which has the smaller density of states, develops the larger superconducting gap, a consequence of the triplon-mediated interband pairing nature. The nonlocal Kondo coupling further introduces momentum-dependent gap anisotropy. The calculated tunneling spectrum faithfully reproduces the experimentally observed double-peak structure and gap hierarchy, providing strong evidence that the triplon-mediated pairing mechanism is the microscopic origin of superconductivity in bilayer nickelates.

2. Squeezing dynamical singlets in bilayer nickelates

Summary: Employing density functional theory combined with cluster dynamical mean-field theory, this study investigates bilayer Ruddlesden-Popper nickelates and finds that their physical properties are primarily governed by interlayer “dynamical singlets” formed by single electrons in the 3z²−r² orbital, which hybridize with itinerant x²−y² planar orbitals. This hybridization responds distinctly to hydrostatic pressure and in-plane compressive strain: strain enhances interlayer correlations, leading to an orbitally selective singlet-pairing Mott mechanism, whereas hydrostatic pressure mainly promotes in-plane itinerancy. This difference explains experimental discrepancies between bulk and strained thin films observed in angle-resolved photoemission spectroscopy and transport measurements. The theoretical framework views this low-energy state as a hybridized system of dynamical singlets and itinerant orbitals, offering a new perspective for understanding superconductivity.