Unified mechanism of charge-density-wave and high-Tc superconductivity protected from oxygen vacancies in bilayer nickelates

摘要

Unconventional charge- and spin-density-wave states are commonly observed in bilayer nickelates, drawing considerable attention due to their proximity to high-transition temperature (T_rmc) superconductivity. However, the nature and origin of these density waves remain poorly understood. Experiments show that the charge-density-wave and spin-density-wave transition temperatures are closely related but distinct, while mean-field-type analyses typically have yielded only a simple spin-density-wave phase. To resolve this key problem, this paper demonstrates that sizeable charge-density-wave instabilities emerge in proportion to spin-density-wave instabilities in La₃Ni₂O₇ due to the paramagnon-interference mechanism, which captures electron correlations beyond mean-field theories. Therefore, (i) the experimental charge- and spin-density-wave coexisting state is naturally explained, and (ii) charge- and spin-density-wave fluctuations cooperatively drive high-T_rmcsuperconductivity. Furthermore, the predicted s-wave superconducting state is robust against the inner-apical oxygen vacancies. We find that the coexistence of charge- and spin-fluctuations is essential in bilayer nickelates, with both playing a cooperative role in mediating high-T_rmcsuperconductivity.

材料

• La₃Ni₂O₇

方法

• paramagnon-interference mechanism
• theoretical analysis

关键词

• charge density wave
• spin density wave
• paramagnon interference
• superconductivity
• oxygen vacancies

亮点

• Charge-density-wave instabilities emerge in proportion to spin-density-wave instabilities due to the paramagnon-interference mechanism, capturing electron correlations beyond mean-field theories.

结论

• Charge- and spin-density-wave fluctuations cooperatively drive high-T_c superconductivity.
• The predicted s-wave superconducting state is robust against inner-apical oxygen vacancies.
• Coexistence of charge- and spin-fluctuations is essential in bilayer nickelates, with both playing a cooperative role in mediating high-T_c superconductivity.

主要论断

• Charge-density-wave instabilities emerge in proportion to spin-density-wave instabilities in La₃Ni₂O₇ due to the paramagnon-interference mechanism, explaining the experimentally observed coexisting CDW+SDW state.
  • 证据: DW equation analysis with AL vertex corrections yields CDW eigenvalue λQ>1 at Q=(π/2,π/2), comparable to SDW Stoner factor.
• CDW and SDW fluctuations cooperatively drive high-T_c superconductivity, yielding s± and dxy pairing states.
  • 证据: SC gap equation including Vbond (from CDW fluctuations) and spin-fluctuation terms gives large λSC for both s-wave and dxy-wave.
• The predicted s-wave superconducting state is robust against inner apical oxygen vacancies, unlike d-wave states.
  • 证据: T-matrix analysis shows λSC for s-wave remains high for nimp up to 0.2, while dxy-wave is strongly suppressed.

研究流程

• Tight-Binding Model Construction — Model captures essential band structure of bilayer nickelates.
  • 材料: La₃Ni₂O₇
  • 方法: DFT (WIEN₂k) + Wannier90
  • 观察: Three Fermi pockets: α, β, γ; γ-pocket from dz2 bonding orbital
• RPA Spin Susceptibility and DW Equation Analysis — CDW instability at Q emerges via paramagnon-interference mechanism, comparable to SDW instability.
  • 材料: La₃Ni₂O₇
  • 方法: RPA for spin susceptibility; linearized DW equation with vertex corrections (AL and MT terms)
  • 观察: Spin susceptibility broad peak near Q and Q₀; CDW eigenvalue λQ > 1 at Q=(π/2,π/2) from PMI mechanism
• Superconducting Gap Equation Analysis — CDW+SDW fluctuations cooperatively drive high-T_c superconductivity, leading to s± or dxy states.
  • 材料: La₃Ni₂O₇
  • 方法: Bethe-Salpeter equation for charge-channel vertex; linearized SC gap equation on FS patches
  • 观察: CDW+SDW fluctuations cooperatively enhance λSC; s-wave and dxy-wave states; λSC > 1 for U>0.99 eV
• Impurity Effect Analysis — The s-wave SC state is protected from inner apical oxygen vacancies.
  • 材料: La₃Ni₂O₇-δ
  • 方法: T-matrix theory for single impurity; oxygen vacancy modeled as wτx in dz2 basis
  • 观察: s-wave SC state robust against vacancies; d-wave state suppressed