Squeezing dynamical singlets in bilayer nickelates

Summary

Employing density functional theory combined with cluster dynamical mean-field theory, this study investigates bilayer Ruddlesden-Popper nickelates and finds that their physical properties are primarily governed by interlayer “dynamical singlets” formed by single electrons in the 3z²−r² orbital, which hybridize with itinerant x²−y² planar orbitals. This hybridization responds distinctly to hydrostatic pressure and in-plane compressive strain: strain enhances interlayer correlations, leading to an orbitally selective singlet-pairing Mott mechanism, whereas hydrostatic pressure mainly promotes in-plane itinerancy. This difference explains experimental discrepancies between bulk and strained thin films observed in angle-resolved photoemission spectroscopy and transport measurements. The theoretical framework views this low-energy state as a hybridized system of dynamical singlets and itinerant orbitals, offering a new perspective for understanding superconductivity.

Materials

• La₃Ni₂O₇

Methods

• DFT
• CDMFT
• quantum Monte Carlo

Keywords

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

Highlights

• 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.

Conclusions

• 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.

Main claims

• Interlayer dynamical singlets formed from 3z²−r² orbitals hybridize with itinerant planar orbitals and control the low-energy physics of bilayer nickelates.
  • Evidence: 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.
  • Evidence: 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.
  • Evidence: 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

Workflow

• low_energy_model_construction — The low-energy physics is described by a bilayer two-orbital Hubbard model.
  • Materials: DFT band structures of La₃Ni₂O₇ under strain and pressure; Hubbard-Kanamori interaction parameters (U=3.1, U'=2.4, J=0.7 eV)
  • Methods: density functional theory; Wannier projection onto Ni 3dx2-y2 and 3z2-r2 orbitals
  • Observations: 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.
  • Materials: TRIQS/CTHYB continuous-time quantum Monte Carlo solver; CDMFT self-consistency loop with four-site cluster
  • Methods: cluster dynamical mean-field theory (CDMFT); bonding-antibonding basis transformation; hybridization expansion CT-QMC
  • Observations: 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.
  • Materials: real-axis self-energy from maximum entropy analytic continuation
  • Methods: evaluation of scattering rate -Im[Σ(ω=0)]
  • Observations: 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.
  • Materials: intersite self-energy Σ₁₂(iω); dimer probability in many-body wavefunction
  • Methods: analysis of self-energy pole structure in bonding-antibonding basis; evaluation of interlayer hopping renormalization and singlet probability
  • Observations: 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