Squeezing dynamical singlets in bilayer nickelates

摘要

采用密度泛函理论结合团簇动力学平均场方法，本文研究了双层Ruddlesden-Popper镍酸盐，发现其物理性质主要受由3z²−r²轨道单电子形成的层间“动力学单态”支配，这些单态与巡游的x²−y²平面轨道发生杂化。该杂化对静水压与面内压应变的响应截然不同：应变增强层间关联，导致轨道选择性单态配对莫特机制，而静水压主要提升面内巡游性。这一差异解释了块材与应变薄膜在角分辨光电子能谱和输运测量中的实验分歧。理论框架将此低能态视为动力学单态与巡游轨道的杂化体系，为后续理解超导电性提供了新思路。

材料

• La₃Ni₂O₇

方法

• DFT
• CDMFT
• quantum Monte Carlo

关键词

• dynamical singlets
• orbital selective mott insulator
• interlayer correlations
• hybridization
• superconductivity
• strange metal
• epitaxial strain
• hydrostatic pressure

亮点

• The discovery that compressive epitaxial strain and hydrostatic pressure drive the system along different directions in the low-energy phase space: strain enhances interlayer singlet formation, while pressure promotes in-plane itinerancy.
• The identification of interlayer 'dynamical singlets' formed from out-of-plane orbitals as a key component of the normal state, extending the orbital-selective Mott picture.
• The theoretical reproduction of the contrasting transport behaviors: linear-in-T resistivity under pressure and quadratic Fermi-liquid behavior under strain, arising from the orbital-dependent self-energy.

结论

• Local and interlayer electronic correlations reshape the normal-state electronic structure of bilayer RP nickelates in qualitatively different ways under hydrostatic pressure and epitaxial strain.
• Hydrostatic pressure primarily enhances in-plane itinerancy, while compressive strain strengthens interlayer correlations and drives the system toward an orbital-selective singlet-paired Mott regime.
• The low-energy state consists of interlayer dynamical singlets formed from out-of-plane orbitals hybridized with itinerant planar orbitals, and this hybridization weakens with increasing compressive strain.
• This framework reproduces the contrasting photoemission and transport signatures observed in strained thin films and bulk single crystals, including orbital-dependent mass renormalizations and linear versus quadratic resistivity.

主要论断

• Interlayer dynamical singlets formed from 3z²−r² orbitals hybridize with itinerant planar orbitals and control the low-energy physics of bilayer nickelates.
  • 证据: CDMFT calculations show dominant two-electron singlet configurations (probability approaching 1) in the dz² subspace under compressive strain,The intersite self-energy develops pole-like structures characteristic of a Hubbard dimer singlet,The dz²-derived band exhibits strong mass renormalization and coherence changes depending on strain/pressure
• Hydrostatic pressure and epitaxial compressive strain tune the normal state along different directions: pressure enhances in-plane itinerancy, strain strengthens interlayer singlet correlations.
  • 证据: Renormalized intralayer hopping t∥ increases substantially with pressure while t⊥ changes little,Renormalized interlayer hopping t⊥ is strongly enhanced by compressive strain, whereas t∥ is only moderately affected,The contrasting evolutions are visualized in a (t∥, t⊥) phase space diagram
• The normal-state transport of bilayer nickelates switches from non-Fermi-liquid (linear-in-T) to Fermi-liquid (quadratic-in-T) between pressurized bulk crystals and strained thin films, as reproduced by scattering rate calculations.
  • 证据: Calculated scattering rate is linear in temperature for the 30 GPa crystal structure,Calculated scattering rate is close to quadratic for the compressively strained structure,Orbital-resolved self-energies show the linear-T behavior originates from dz² fluctuations when the flat band approaches the Fermi level

研究流程

• low_energy_model_construction — The low-energy physics is described by a bilayer two-orbital Hubbard model.
  • 材料: DFT band structures of La₃Ni₂O₇ under strain and pressure; Hubbard-Kanamori interaction parameters (U=3.1, U'=2.4, J=0.7 eV)
  • 方法: density functional theory; Wannier projection onto Ni 3dx2-y2 and 3z2-r2 orbitals
  • 观察: four-orbital low-energy Hamiltonian with interlayer hopping via apical oxygen
• many_body_CDMFT_solution — Many-body calculations capture experimental ARPES mass renormalizations and reveal distinct orbital-selective behavior.
  • 材料: TRIQS/CTHYB continuous-time quantum Monte Carlo solver; CDMFT self-consistency loop with four-site cluster
  • 方法: cluster dynamical mean-field theory (CDMFT); bonding-antibonding basis transformation; hybridization expansion CT-QMC
  • 观察: orbital-dependent mass renormalizations (m*/m ≈ 3-5 for dz2 band, ≈2 for dx2-y2 bands); dynamical singlet formation in dz2 orbitals under compressive strain
• transport_scattering_calculation — The transport dichotomy between strange-metal pressurized crystals and Fermi-liquid strained films is reproduced.
  • 材料: real-axis self-energy from maximum entropy analytic continuation
  • 方法: evaluation of scattering rate -Im[Σ(ω=0)]
  • 观察: linear-in-T scattering rate for pressurized structure; quadratic-in-T scattering rate for compressively strained structure
• interlayer_correlation_analysis — Compressive strain drives the system into an orbital-selective singlet-paired Mott regime, while hydrostatic pressure promotes in-plane itinerancy.
  • 材料: intersite self-energy Σ₁₂(iω); dimer probability in many-body wavefunction
  • 方法: analysis of self-energy pole structure in bonding-antibonding basis; evaluation of interlayer hopping renormalization and singlet probability
  • 观察: pole-like self-energy for compressive strain at ±U/2 and ±(U+2J); dominant two-electron singlet in 3z²−r² orbitals under compressive strain; renormalized interlayer hopping t⊥ strongly enhanced by strain; intralayer hopping t∥ enhanced by pressure