What Can ABACUS Do Too? | Explain the Formation Mechanism of Conductive Filaments in RRAM Based on TiOx/MoS2–xOx Structure Induced by S Vacancies
The research team led by Associate Professor Kun Cao from the School of Physics, Sun Yat-sen University, has made significant progress in the study of the magnetism of bilayer nickel-based superconductor La₃Ni₂O₇₋δ under normal temperature and pressure. Through density functional theory (DFT) calculations and Monte Carlo simulations, this research has for the first time revealed the regulatory mechanism of oxygen vacancies on the magnetic ground state and phase transition temperature (TSDW) of the material, providing a new perspective for understanding the relationship between high-temperature superconductivity and magnetism in nickel oxides.
This achievement was published in the journal npj Quantum Materials (Paper link: https://www.nature.com/articles/s41535-025-00740-z).
Research Background
Recently, the discovery of high-temperature superconductivity at 80 K in La₃Ni₂O₇₋δ under high pressure has attracted widespread attention. Experiments show that the ground state of La₃Ni₂O₇₋δ under normal pressure may be a spin density wave (SDW) phase, but its magnetic configuration has not been fully determined. In addition, the impact of oxygen vacancies on magnetism remains unclear. Understanding its regulatory mechanism on the magnetic ground state is crucial for revealing the relationship between magnetism and superconductivity.
Research Results
Exploration of the Magnetic Ground State and Magnetic Mechanism
Through DFT+U calculations, this study found that when U ≤ 2 eV, the magnetic ground state of La₃Ni₂O₇₋δ is a double spin stripe (DSS), and this result was further verified by HSE06 hybrid functional calculations implemented by ABACUS.
Figure 1 | Crystal structure of La₃Ni₂O₇₋δ and DFT + Ueff calculation results. (a) Schematic diagram of the crystal structure, and the red dotted circle represents the inner apical oxygen vacancy. (b) and (c) are the total energy and average magnetic moment under different Ueff values respectively.
Through DFT calculations, the study found that there are two types of Ni atoms with different magnetic moments in the ground state DSS of La₃Ni₂O₇₋δ.
By calculating and analyzing the magnetic interactions using the classical Heisenberg model, the dominant magnetic interaction was determined to be the interlayer interaction AFM J₃, which is consistent with experimental findings.
The study found that the inner apical oxygen vacancy (as shown in Figure 4a) is the most stable and will cause the adjacent Ni atoms to lose magnetism, forming a third type of Ni atom.
Figure 2 | Magnetic configurations and magnetic interactions of La₃Ni₂O₇. (a) DSS configuration, (b) SCS configuration, (c) Schematic diagram of magnetic exchange paths, (d) Comparison of different magnetic configurations.
Core Finding: How Do Oxygen Vacancies "Rewrite" the Magnetic Script?
Under normal pressure, La₃Ni₂O₇ with a perfect stoichiometric ratio (δ = 0) tends to have DSS magnetic order, and its spin wave vector is (Q = 0.25, 0.25).
After the introduction of oxygen vacancies, the magnetic moments of neighboring nickel atoms disappear, forming charge sites, and the magnetic ground state gradually transitions to a spin-charge stripe (SCS), and finally evolves into a short-range ordered spin glass-like state at δ = 0.5.
Figure 3 | Phase diagram and typical magnetic ground states of La₃Ni₂O₇₋δ. (a) Phase diagram, (b)–(d) Schematic diagrams of the magnetic ground states when δ is 0.25, 0.375 and 0.5 respectively.
Further calculation and analysis of the density of states determined the electron occupation states of the three types of Ni under O vacancies (as shown in Figure 4b).
The study found that after the introduction of oxygen vacancies, the energy of the dz² orbital of Ni3 decreased significantly, resulting in its electron configuration changing to low-spin Ni²⁺ (t₂g⁶dz₂²), and the magnetic moment disappeared, further disrupting the interlayer superexchange interaction.
Figure 4 | Electronic structure of La₃Ni₂O₇₋δ. (a) Density of states of Ni1, Ni2 and Ni3, (b) Schematic diagram of electron configurations, (c) Magnetic moment and charge order in the DSS phase, (d) Charge order in the double spin/charge stripe phase, (e) Electronic band structure.
Oxygen vacancies significantly affect the magnetism, charge, and orbital order of La₃Ni₂O₇₋δ by regulating the electron configuration of Ni and the interlayer interaction, revealing its complex electron correlation behavior and providing important theoretical support for understanding the physical properties of nickel oxides.
At the same time, oxygen vacancies significantly reduce TSDW by diluting the exchange interaction and make the spin waves more consistent with the experimental measurement results (as shown in Figure 5d).
Figure 5 | Magnetic excitation spectra of La₃Ni₂O₇₋δ. (a)–(d) are the spin wave calculation results under different oxygen vacancy distributions respectively, and the red dots are the experimental data.
Scientific Significance: Filling a Piece in the "Puzzle" of High-Temperature Superconductivity
The high-pressure superconducting phase of La₃Ni₂O₇₋δ with Tc ~ 80 K has attracted global attention in recent years, but its magnetism and formation mechanism under normal pressure have long been controversial. This study has for the first time constructed a quantitative phase diagram of oxygen vacancy concentration and magnetic order, revealing the competitive coexistence of DSS and SCS, and providing a unified theoretical framework for contradictory experimental observations.
Research Team and Acknowledgments
The only corresponding author of the paper is Associate Professor Kun Cao from the School of Physics, Sun Yat-sen University. This research was supported by projects such as the National Natural Science Foundation of China, the Natural Science Foundation of Guangdong Province, and the Key Laboratory of Guangdong Province. The collaborating institutions include the University of Science and Technology of China and the Hefei Comprehensive National Science Center.