Daily Overview: Today’s highlights focus on a multi-angle analysis of density waves, electronic structure characterization, and the superconducting pairing mechanism in hybrid Ruddlesden–Popper nickelates. In [1], ultrafast optical experiments reveal strong coupling between the density wave and the lattice in La₄Ni₃O₁₀ single crystals and demonstrate that ultrafast photoexcitation can non‑thermally suppress this order parameter, offering a new approach for the non‑equilibrium control of quantum states in nickelates. [2] develops a scanning tunneling microscopy/spectroscopy electronic theory for superconducting bilayer nickelate thin films, pointing out that the multi‑orbital character and the tip‑height‑dependent changes in spectral weight can distinguish the controversial γ‑band and β‑band coherence peaks, and that quasiparticle interference patterns can identify s‑wave or d‑wave pairing symmetry. [3] constructs a pairing model mediated by local d₃z²−r² spin‑triplet excitations (triplons), predicts an interband s±‑wave superconducting gap, and successfully explains the experimentally observed anomalous gap magnitude and the double‑peak structure in tunneling spectra, strongly supporting the triplon mechanism as the microscopic origin of unconventional superconductivity in nickelates. arXiv submission processing window: 2026-07-01 00:00 to 2026-07-01 00:00 UTC.

1. Nonthermal melting and density wave instability coupled to the lattice in La$_4$Ni$_3$O$_{10}$

Summary: Using ultrafast optical spectroscopy on La₄Ni₃O₁₀ single crystals, researchers observed an abrupt change in quasiparticle relaxation dynamics at the density-wave transition temperature of approximately 136 K, revealing the opening of a strongly coupled energy gap of about 52 meV. Multiple coherent phonon modes, including Ag modes near 3.88, 5.28, and 2.09 THz, exhibited mode-selective anomalies across the transition, with the renormalization behavior of the 3.88 THz phonon in particular shifting from conventional anharmonic decay at high temperatures to pronounced hardening at low temperatures, indicating strong coupling between the density-wave instability and lattice degrees of freedom and suggesting that electron–phonon interactions likely play a critical role. Under high excitation fluence, the density wave is suppressed non-thermally, yielding a temperature–fluence phase diagram that resembles the pressure-tuning behavior, though the gap remains relatively stable, leading to an increased coupling ratio. These findings establish the density wave in La₄Ni₃O₁₀ as a lattice-entangled instability involving multiorbital physics and confirm that ultrafast photoexcitation can serve as a non-equilibrium control parameter to effectively suppress density-wave order in nickelates.

2. Electronic theory for scanning tunneling microscopy spectra in bilayer nickelate thin films

Summary: This paper theoretically analyzes scanning tunneling microscopy spectra of superconducting bilayer nickelate films using a two-orbital bilayer model based on first-principles Wannier functions and the continuous Green’s function method. The study finds that the multi-orbital character and the spatial anisotropy of Wannier functions render the local density of states highly sensitive to the tip position: as the tip height increases, the relative weights of coherence peaks from different bands change significantly, thereby enabling distance-dependent measurements to distinguish the orbital origins of the controversial γ-band and β-band coherence peaks. Furthermore, in impurity-containing systems, quasiparticle interference patterns can clearly resolve the symmetry of s-wave and d-wave superconducting order parameters. This work provides explicit theoretical guidance for experimentally identifying the band attribution of superconducting gaps and the pairing symmetry.

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

Summary: We construct a bilayer nickelate superconducting model in which localized d₃z²₋r² spins coexist with itinerant dₓ²₋y² bands. Strong interlayer coupling drives the local magnetic moments into a singlet ground state, and virtual singlet-triplet excitations (“triplons”) serve as the pairing mediator. This mechanism yields interband s±-wave pairing, with opposite signs of the order parameters on the α and β bands. The theoretically calculated tunneling spectrum reproduces the experimentally observed double-peak structure and explains the counterintuitive feature that the α band, despite its smaller density of states, exhibits a larger gap, as well as the gap anisotropy arising from nonlocal Kondo coupling. These results strongly support that triplon-mediated pairing is the microscopic origin of unconventional superconductivity in bilayer nickelates.