Daily Overview: Today’s highlight work focuses on deepening the understanding of the electronic structure of hybrid Ruddlesden-Popper nickelates. In [1], using the newly discovered nickel-based superconductor La₃Ni₂O₇ as an example, the study reveals the “orbital dimerization” mechanism underlying its first-order structural phase transition. The authors find that standard DFT and LDA+DMFT methods fail to reproduce the abruptness of the phase transition due to neglecting key interatomic correlations. By constructing a many-body effective Hamiltonian involving low-energy active orbitals and performing exact diagonalization, they point out that when the Ni-O-Ni bond angle approaches linearity, the strong antiferromagnetic superexchange interaction between interlayer Ni orbitals forms spin-singlet bonding, leading to a sharp drop in total energy. This generates a new local energy minimum at a specific configuration, coexisting with the tilted configuration, perfectly explaining the first-order phase transition characteristics and the coexistence of high- and low-pressure structures observed in experiments. This mechanism not only alters the lattice bonding properties but also qualitatively changes the low-energy electronic structure (e.g., the emergence of superconductivity). The established DFT plus many-body correction computational framework provides key microscopic insights into understanding the structure-electronic property correlations in ionic materials containing open-shell d/f electrons. arXiv submission processing window: 2026-03-16 00:00 to 2026-03-16 00:00 UTC.

1. Orbital dimerization-induced first-order structural phase transition: a case study in La$_3$Ni$_2$O$_7$

Summary: This study investigates the newly discovered nickel-based superconductor La₃Ni₂O₇ and reveals the “orbital dimerization” mechanism underlying its first-order structural phase transition. The authors note that standard density functional theory (DFT) and the LDA+DMFT method incorporating intra-atomic correlations fail to reproduce the abruptness of this transition because they neglect crucial inter-atomic correlations. By constructing a many-body effective Hamiltonian that includes low-energy active orbitals and performing exact diagonalization, they find that when the Ni-O-Ni bond angle approaches linearity, strong antiferromagnetic superexchange interactions emerge between interlayer Ni orbitals, forming spin-singlet bonds that cause a sharp drop in total energy. This energy reduction creates a new local energy minimum at a specific configuration that coexists with the tilted configuration, perfectly explaining the first-order transition characteristics observed in experiments as well as the coexistence of high- and low-pressure structures. This mechanism not only alters the lattice bonding properties but also leads to qualitative changes in the low-energy electronic structure, such as the emergence of superconductivity. The computational framework of DFT plus many-body corrections established in this work is universal and applicable to ionic materials containing open-shell d/f electrons, providing key microscopic insights into understanding the structure–electronic property relationships in such systems.