Tunable superconductivity and spin density wave in La₃Ni₂O₇/LaAlO₃ thin films

摘要

通过第一性原理计算与奇异模式泛函重整化群方法，系统研究了La₃Ni₂O₇/LaAlO₃薄膜中层间镍-镍距离对基态的影响。结果显示，较小的层间距导致C型自旋密度波（层间铁磁耦合），较大的层间距则产生G型自旋密度波（层间反铁磁耦合），而在这两种相之间，出现了以镍3d₃z^2-ᵣ²轨道配对为主导的s±波超导态。该结果解释了实验中薄膜在常压下超导的成因，并预测施加压力会降低超导转变温度，直至系统进入C型自旋密度波。这一预言若经实验验证，将为理解该体系中电子关联的本质提供深刻见解，因为C型自旋密度波在巡游电子图像中自然实现，而在局域磁矩图像中则难以形成（后者层间自旋始终反铁磁耦合）。

材料

• La₃Ni₂O₇/LaAlO₃

方法

• first-principles calculations
• singular-mode functional renormalization group (SM-FRG)
• DFT

关键词

• c type spin density wave
• g type spin density wave
• s± wave superconductivity
• interlayer nickel nickel distance

亮点

• The study predicts that applied pressure will decrease the superconducting transition temperature until the system enters the C-type spin density wave.
• The C-type SDW is naturally achieved in the itinerant picture, whereas it would be difficult in the local moment picture.

结论

• For smaller interlayer distance, the ground state is a C-type spin density wave (interlayer ferromagnetic), while a larger distance yields a G-type spin density wave (interlayer antiferromagnetic).
• Between these phases, s±-wave superconductivity emerges, dominated byNi 3d_z2 orbital pairing.

主要论断

• The ground state of La₃Ni₂O₇/LAO thin films tunes from C-type SDW through s±-wave SC to G-type SDW with increasing interlayer Ni-Ni distance
  • 证据: SM-FRG calculations show that for smaller d_Ni the leading instability is C-type SDW with interlayer ferromagnetic coupling, for larger d_Ni it is G-type SDW with interlayer antiferromagnetic coupling, and at intermediate d_Ni an s±-wave SC state emerges dominated by dz2 orbital pairing

研究流程

• First-principles calculations — Electronic structure depends sensitively on d_Ni
  • 材料: La₃Ni₂O₇ thin film with in-plane lattice constant 3.78 Å
  • 方法: DFT using VASP; Maximally localized Wannier functions
  • 观察: Band structure, Fermi surfaces, density of states
• Singular-mode functional renormalization group (SM-FRG) — Ground state evolves from C-type SDW to SC to G-type SDW with increasing d_Ni
  • 材料: Tight-binding model from DFT
  • 方法: SM-FRG including Coulomb interactions (U=5 eV, J_H=1 eV)
  • 观察: For small d_Ni: C-type SDW (interlayer FM); for large d_Ni: G-type SDW (interlayer AFM); intermediate: s±-wave SC