Source capture
Authors Xingchen Shen, Wei Ku
arXiv / Publisher https://arxiv.org/abs/2603.12924
PDF https://arxiv.org/pdf/2603.12924
Relevance score 5.232
Primary category cond-mat.str-el
Published 2026-03-16
Research paradigm Theoretical
Sample form Unknown

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.

Materials

Methods

Keywords

  • orbital dimerization
  • first order structural phase transition
  • spin singlet bond
  • inter atomic correlation
  • ni o ni bond angle
  • spin fractionalization

Highlights

  • This is the first identification of a general orbital dimerization mechanism for pressure-induced first-order structural phase transitions.
  • The developed many-body framework improves DFT total energy by including inter-atomic correlations.
  • The mechanism explains the coexistence of tilted and straight octahedra structures over a wide pressure range.

Conclusions

  • The first-order structural phase transition in La3Ni2O7 is reproduced by incorporating inter-atomic correlations via a full many-body treatment of low-energy active orbitals.
  • The mechanism involves orbital dimerization through spin-singlet bond formation, leading to abrupt energy reduction.
  • State-of-the-art DFT and LDA+DMFT methods fail to capture the first-order nature because they omit inter-atomic correlations.
  • The orbital dimerization qualitatively alters the low-energy electronic structure, effectively fractionalizing Ni2+ ionic spin to 1/2 and producing cuprate-like dynamics.

Main claims

  • Standard LDA and LDA+DMFT fail to reproduce the first-order nature of the structural transition; inter-atomic correlation is essential.
    • Evidence: Abstract,Full text: LDA and LDA+DMFT display a smooth behavior… completely misses the first-order nature.
  • A full many-body treatment reveals an abrupt energy reduction due to inter-atomic orbital dimerization through spin-singlet bonding.
    • Evidence: Abstract,Full text: The energy improvement near 180° is associated with the emergence of strong inter-layer superexchange and corresponding spin-singlet correlation.
  • The orbital dimerization mechanism qualitatively alters low-energy electronic structure, fractionalizing Ni2+ spin to 1/2.
    • Evidence: Abstract,Full text: Ni2+ ionic spins are effectively fractionalized from 1 into 1/2, leaving only half-filled effective orbital active.

Workflow

  • DFT_calculation
    • Materials: La3Ni2O7
    • Methods: LDA; LDA+DMFT; LDA+U
    • Observations: smooth energy variation with tilt angle in LDA and DMFT
  • many_body_model_construction
    • Methods: exact diagonalization of effective Hamiltonian for active orbital subspace
  • energy_analysis
    • Methods: total energy calculation with many-body correction; double counting removal
    • Observations: double-minimum structure in total energy; strong energy reduction near 180° bond angle
  • interpretation — First-order structural transition is due to inter-atomic orbital dimerization via spin-singlet bond formation.
    • Methods: spin-singlet bond analysis
    • Observations: strong interlayer superexchange at straightened octahedra