Electronic Reconstruction across the Tilt-Free Transition in La₃Ni₂O₇

Summary

This work uses ambient-pressure broadband infrared-to-visible reflectivity spectra (15 meV–3.2 eV) combined with high-temperature Raman measurements to investigate the electronic response of La₃Ni₂O₇ before and after its transition at approximately 544 K from a NiO₆ octahedrally tilted phase to an untilted structure. The results show that spectral weight undergoes pronounced redistribution over an exceptionally broad energy range, transferring from 1–3 eV excitations to low-energy excitations below 1 eV; in particular, two low-energy interband excitations gradually approach and merge upon entering the untilted phase, indicating a substantial reconstruction of the finite-energy electronic structure. Drude–Lorentz analysis further attributes these changes mainly to finite-energy excitations rather than the Drude response, and points to reconstruction of bilayer Ni 3d_z²-derived electronic states, whose interlayer coupling is considered central to the superconducting mechanism of La₃Ni₂O₇. On this basis, the authors argue that the transition to the untilted phase can directly reorganize electronic degrees of freedom relevant to high-temperature superconductivity and provide an ambient-pressure reference for the electronic structure of the superconducting state.

Materials

• La₃Ni₂O₇

Methods

• High-temperature broadband infrared-to-visible reflectivity microspectroscopy
• Synchrotron infrared spectromicroscopy
• High-temperature Raman spectroscopy
• Kramers-Kronig analysis
• Drude-Lorentz fitting with RefFIT

Keywords

• high tc superconductivity
• tilt free transition
• structural transition
• spectral weight redistribution
• interband excitations
• bilayer ni 3d_z2 derived electronic states
• interlayer coupling
• nio6 octahedral tilts
• electronic reconstruction
• drude response
• fano like asymmetry

Highlights

• Ambient-pressure broadband reflectivity from 15 meV to 3.2 eV was used to track the electronic response across the tilt-free structural transition.
• The low-energy spectral-weight gain does not fully compensate the high-energy loss within 0–26,000 cm^-1, indicating that the redistribution extends into the ultraviolet above 3.2 eV.
• Complementary high-temperature Raman spectra show Fano-like asymmetry and the disappearance of tilt-related NiO₆ modes between 543 and 545 K, coincident with the structural transition.
• High-pressure and high-temperature routes to the same tilt-free structure differ: pressure contracts the lattice and strongly enhances the Drude weight, whereas heating expands the lattice and leaves the Drude response nearly unchanged.
• The observed merging of two low-energy interband excitations and the splitting of the lowest-energy mode are opposite to some high-pressure band calculations for the bonding and antibonding states.
• The study provides direct optical evidence for reconstruction of the bilayer Ni 3d_z2-derived electronic sector across the tilt-free transition.

Conclusions

• The temperature-driven transition to the tilt-free phase at T_st ≈ 544 K in La₃Ni₂O₇ produces a pronounced redistribution of electronic spectral weight over an exceptionally broad energy range.
• Spectral weight is transferred from excitations between 1 and 3 eV toward low-energy excitations below 1 eV.
• Two low-energy interband excitations progressively converge and merge upon entering the tilt-free phase, revealing a substantial reconstruction of the finite-energy electronic structure.
• The changes are dominated by finite-energy interband excitations rather than the Drude response, with the largest low-energy spectral-weight gain associated with interband components.
• The optical response points to a reconstruction of the bilayer Ni 3d_z2-derived electronic states; suppression of NiO₆ octahedral tilts modifies the Ni-Oap-Ni geometry, the 3d_z2-O hybridization, and apical-oxygen-mediated interlayer coupling.
• The tilt-free transition provides an ambient-pressure reference for the electronic structure of the superconducting state and links lattice distortion, electronic structure, and interlayer interactions implicated in high-T_c superconductivity.

Main claims

• Across the tilt-free transition at T_st ≈ 544 K, spectral weight is redistributed over an exceptionally broad energy range, transferring from 1-3 eV excitations to low-energy excitations below 1 eV.
  • Evidence: Broadband reflectivity from 15 meV to 3.2 eV at ambient pressure,Kramers-Kronig-derived optical conductivity,Differential optical conductivity and integrated spectral weight show low-energy gain and high-energy loss
• Two low-energy interband excitations progressively converge and merge upon entering the tilt-free phase, revealing substantial reconstruction of the finite-energy electronic structure.
  • Evidence: Drude-Lorentz fits show resonance frequencies of two low-energy components approach and become nearly degenerate above T_st,Component-resolved spectral-weight changes are dominated by these finite-energy oscillators
• The optical changes are dominated by finite-energy interband excitations rather than the Drude free-carrier response.
  • Evidence: Drude-Lorentz decomposition with one Drude term and six Lorentz oscillators,The Drude spectral-weight gain is comparatively small while two Lorentz components account for the largest low-energy spectral-weight enhancement
• The observed reconstruction points to bilayer Ni 3d_z2-derived electronic states, with modified Ni-Oap-Ni geometry and interlayer coupling.
  • Evidence: Comparison with orbital-resolved band structure and optical calculations,Suppression of NiO₆ octahedral tilts changes Ni-O geometry and apical-oxygen-mediated interlayer coupling,Fitted low-energy excitations assigned to bonding and antibonding branches of Ni 3d_z2-derived states
• The tilt-free transition provides an ambient-pressure reference linking lattice distortion to electronic degrees of freedom relevant to high-T_c superconductivity in La₃Ni₂O₇.
  • Evidence: The transition reconstructs the Ni 3d_z2 orbital sector central to proposed interlayer pairing mechanisms,Ambient-pressure transition offers a reference for the pressure-induced superconducting regime

Workflow

• sample_preparation — La₃Ni₂O₇ single crystals were prepared and mounted for ambient-pressure high-temperature optical and Raman measurements.
  • Materials: La₃Ni₂O₇ single crystals; ab plane of La₃Ni₂O₇; gold reference; aluminum reference
  • Methods: single-crystal synthesis and characterization; mounting on a heating stage; temperature control from 296 to 723 K
  • Observations: La₃Ni₂O₇ single crystals were synthesized and characterized and then mounted on a heating stage for temperature-dependent measurements.
• measurement — Temperature-dependent broadband reflectivity and Raman data track the structural transition across T_st ≈ 544 K.
  • Materials: La₃Ni₂O₇ single crystal; gold infrared reference; aluminum near-infrared/visible reference
  • Methods: broadband infrared-to-visible reflectivity microspectroscopy; four complementary spectral ranges; calibration using gold and aluminum references at each temperature; repeated heating and cooling cycles; complementary high-temperature Raman spectroscopy with a 532 nm laser
  • Observations: Reflectivity spectra were reproducible over several heating and cooling cycles with no sample degradation.; Raman modes redshift and disappear between 543 and 545 K, coincident with the structural transition at T_st ≈ 544 K.
• analysis — The tilt-free transition drives a broadband spectral-weight redistribution dominated by finite-energy interband excitations rather than the Drude response.
  • Materials: reflectivity spectra; optical conductivity spectra
  • Methods: Kramers-Kronig analysis; differential optical conductivity relative to 296 K; integration of spectral weight over selected frequency ranges; Drude-Lorentz fitting with one Drude term and six Lorentz oscillators using RefFIT; component-resolved integration; cumulative spectral-weight change analysis
  • Observations: Spectral weight below 8000 cm-1 increases with a step-like enhancement around T_st; spectral weight between 8000 and 26000 cm-1 decreases continuously.; The low-energy spectral-weight gain does not fully compensate the high-energy loss within the measured range.; Two finite-energy oscillators converge and become nearly degenerate above T_st; their spectral weights increase strongly, while the Drude contribution changes little.
• interpretation — The tilt-free transition reorganizes the bilayer Ni 3d_z2-derived electronic sector, modifying interlayer coupling implicated in superconductivity.
  • Materials: experimental optical spectra; previous orbital-resolved electronic band structure calculations; previous ARPES results; previous optical calculations
  • Methods: comparison of fitted excitation energies with band-structure and orbital-resolved calculations; assignment of optical excitations to Ni 3d_z2-derived bonding and antibonding bands
  • Observations: The observed convergence and spectral-weight enhancement of low-energy excitations are consistent with reconstruction of bilayer Ni 3d_z2-derived electronic states.; High-temperature ambient-pressure behavior differs from high-pressure behavior: bonding bands become more split and antibonding branches become degenerate, unlike high-pressure calculations.