Daily Overview: Today’s highlighted work focuses on deepening the understanding of pressure-induced structural phase transitions and oxygen-vacancy-related transport anomalies in nickelate superconductors. Synchrotron X-ray diffraction studies reveal that the bilayer nickelate La₂SmNi₂O₇ undergoes sequential monoclinic→orthorhombic→tetragonal structural transformations across the pressure range where superconductivity emerges, without the involvement of a displacive charge density wave. This provides precise constraints on bond angles and lattice parameters for the superconducting state. Transport theory work demonstrates that the sign reversal of the Hall coefficient in nickelate thin films originates from orbital-selective scattering by oxygen vacancies—in-plane oxygen vacancies strongly suppress the d_x²−y² orbital transport channel, causing the Hall coefficient to switch from negative to positive, whereas apical oxygen vacancies act in the opposite manner. This offers a microscopic picture to unify the diverse Hall responses observed experimentally as a function of oxygen stoichiometry. arXiv submission processing window: 2026-07-07 00:00 to 2026-07-07 00:00 UTC.

1. Pressure-Driven Structural Transitions without a Displacive Charge-Density Wave in La$_2$SmNi$_2$O$_7$

Summary: This study employs synchrotron X-ray diffraction to systematically investigate the structural evolution of the bilayer nickelate La₂SmNi₂O₇ under low temperature and high pressure. At ambient conditions, single-crystal diffraction reveals a new monoclinic superstructure (space group P2₁/a) with a c-axis doubling primarily driven by antiferrodistortive oxygen displacements; no satellite reflections associated with charge-density-wave order are detected, indicating that any displacive charge ordering, if present, has an amplitude below a few thousandths of an ångström. Under applied pressure, a sequence of structural transitions is observed at room temperature using both powder and single-crystal diffraction: a monoclinic-to-orthorhombic transition at approximately 15 GPa, followed by a further transition to tetragonal symmetry near 21 GPa, with the intermediate orthorhombic phase persisting stably over a finite pressure interval. In the pressure–temperature regime where superconductivity emerges, high-quality single-crystal data enable structural refinement and provide precise lattice parameters and bond angles, establishing a structural basis for understanding the onset of superconductivity. The results demonstrate that the pressure-driven structural transformations in La₂SmNi₂O₇ occur without the participation of a displacive charge-density wave, and the successive symmetry changes impose crucial constraints on theoretical models aimed at exploring the interplay of charge, lattice, and magnetism.

2. Hall Coefficient Sign Reversal Driven by Orbital-Selective Oxygen-Vacancy Scattering in Nickelate Films

Summary: Combining a correlated multi-orbital quasiparticle model derived from DFT+CDMFT with the T-matrix method, this study treats oxygen vacancy scattering within a semiclassical Boltzmann transport framework and reveals the microscopic origin of the Hall coefficient sign reversal in bilayer nickelate thin films. Multiband compensation alone is insufficient to explain the phenomenon; in-plane oxygen vacancies strongly suppress the transport channel dominated by the d_{x^2-y^2} orbital through orbital-selective scattering, driving the Hall coefficient across zero to become positive, whereas apical oxygen vacancies tend to make the Hall coefficient more negative. This pocket-resolved and orbital-selective scattering mechanism demonstrates that oxygen vacancies act not only as electron doping sources but also as active scattering centers whose spatial distribution directly controls the normal-state transport behavior, providing a theoretical framework for a unified understanding of the diverse Hall responses observed experimentally as a function of oxygen stoichiometry.