Daily Overview: Today’s highlights focus on deepening the understanding of the electronic structure of mixed Ruddlesden-Popper nickelates. In [1], a study based on high-brightness synchrotron radiation X-ray diffraction precisely determined the polar structure (space group Ima2) of La₃Ni₂O₇, revealing a previously overlooked glide mirror symmetry breaking. By analyzing bond length differences, it confirmed a checkerboard charge order state at nickel sites, which competes with pressure-induced superconductivity. This finding provides a critical structural basis for understanding the phase competition mechanism in bilayer nickelates. In [2], from a theoretical perspective, a systematic study of the bilayer Kondo lattice model was conducted using single-site dynamical mean-field theory. It was found that interlayer spin coupling or hole doping can drive a non-Fermi liquid critical point, separating the overdoped Fermi liquid from the underdoped pseudogap metal (i.e., the “second Fermi liquid”). Mapping this model to La₃Ni₂O₇, it predicts that current experimental samples lie in the overdoped Fermi liquid region, while electron doping is expected to access the pseudogap phase and the non-Fermi liquid critical regime, offering theoretical guidance for exploring anomalous metallic behavior in such materials. These two works deepen the understanding of charge order, pseudogap, and non-Fermi liquid behavior in nickel-based superconductors from experimental and theoretical perspectives, respectively. arXiv submission processing window: 2026-03-27 00:00 to 2026-03-27 00:00 UTC.

1. Polar, checkerboard charge order in bilayer nickelate La3Ni2O7

Summary: This study employs high-brightness synchrotron X-ray diffraction for precise structural analysis of high-quality single crystals of the bilayer nickelate La₃Ni₂O₇. Using a large dynamic range detector, we successfully resolved previously overlooked weak diffraction signals, whose intensities are nearly four orders of magnitude weaker than the main Bragg reflections. These observations indicate the presence of glide mirror symmetry breaking in the crystal, leading to a polar structure (space group Ima2) instead of the previously assumed centrosymmetric model (Fmmm). Further structural refinement reveals two inequivalent nickel sites with significantly different Ni–O bond lengths. Combined with bond valence sum calculations, this suggests a checkerboard-like charge ordering of nickel sites, which, together with oxygen octahedral tilting, endows the crystal with polarity. The charge-ordered phase is structurally analogous to the polar state observed in bilayer manganites. This study establishes the polar charge-ordered state of La₃Ni₂O₇ at ambient pressure, indicating its competition with pressure-induced superconductivity, and provides critical structural insights for understanding phase competition mechanisms and the origin of pressure-induced superconductivity in bilayer nickelates.

2. Pseudogap and Non-Fermi-liquid criticality in double Kondo model for bilayer nickelates

Summary: This study systematically investigates the bilayer Kondo lattice model using single-site dynamical mean-field theory (DMFT) to explore the phase diagram of the normal state of bilayer nickelates. In the absence of interlayer tunneling, a non-Fermi-liquid critical point tuned by interlayer spin coupling or hole doping is identified, separating the standard Fermi liquid in the overdoped region from a pseudogap metal in the underdoped region. This pseudogap phase, termed the “second Fermi liquid,” is characterized by small hole pockets and violates the perturbative Luttinger theorem, yet exhibits no symmetry breaking or fractionalization; its behavior resembles that of a heavy Fermi liquid with small quasiparticle residues and large effective masses. Furthermore, an intuitive analytical description of the pseudogap and ground-state wavefunction is provided within the ancilla fermion framework, where the ancilla fermion is interpreted as a spin polaron, and the Kondo resonance peak of this composite fermion is directly shown in DMFT calculations. Extending the analysis to finite interlayer tunneling, the study applies the results to the bilayer nickelate La₃Ni₂O₇, proposing that current experimental samples (x≈0.5) lie in the overdoped Fermi liquid region, while electron doping may drive the system into the pseudogap phase and the non-Fermi-liquid critical regime, offering theoretical predictions for understanding anomalous metallic behavior in such materials.