High-energy electronic excitations in La₃Ni₂O₇ by time-resolved optical spectroscopy

Summary

This study employs time-resolved optical spectroscopy to investigate the ultrafast dynamics of high-energy electronic excitations in bilayer nickelate La₃Ni₂O₇ from 10 K to room temperature at ambient pressure. Two high-energy electronic excitations originating from distinct interband transitions are identified at approximately 1.8 eV and 2.4 eV, revealing different density wave (DW) gaps of about 54 meV and 67 meV, respectively. The relaxation dynamics of these two excited states are well described by the Rothwarf-Taylor model. Additionally, four coherent Raman-active phonon modes are observed, exhibiting varying coupling strengths to the different electronic excitations. The phonon softening upon heating from about 100 K to room temperature can be explained by a semi-quantitative model incorporating thermal expansion and anharmonic phonon-phonon coupling, while the deviation of measured phonon frequencies from the model fit at low temperatures suggests an additional contribution from electron-phonon coupling. This work directly demonstrates the complex gap structure and phonon dynamics in this material, providing key insights into its density wave mechanism and many-body effects.

Materials

• La₃Ni₂O₇ bulk crystals

Methods

• time-resolved optical spectroscopy
• Rothwarf-Taylor model

Keywords

• density wave gaps
• coherent phonon modes
• electron phonon coupling

Highlights

• Four coherent Raman-active phonon modes are observed with varying coupling strengths.
• Deviation of phonon frequencies from model fits at low temperatures suggests additional electron-phonon coupling.

Conclusions

• Two high-energy electronic excitations at ≈1.8 eV and ≈2.4 eV reveal density wave gaps of approximately 54 meV and 67 meV.
• The relaxation dynamics are well described by the Rothwarf-Taylor model.

Main claims

• Two distinct density-wave gaps of ≈54 meV and ≈67 meV exist in La₃Ni₂O₇
  • Evidence: Temperature-dependent red shifts of electronic excitations at ≈1.8 eV and ≈2.4 eV,Relaxation dynamics described by Rothwarf-Taylor model
• Four coherent Raman-active phonon modes exhibit varying coupling to electronic excitations and reveal electron-phonon coupling
  • Evidence: FFT analysis identifies four modes P₁-P₄,Phonon softening fits model with thermal expansion and anharmonicity, but deviations at low T indicate additional electron-phonon coupling

Workflow

• Sample preparation
  • Materials: La₃Ni₂O₇ bulk crystal
  • Methods: Growth; Cryostat mounting
• Time-resolved optical spectroscopy measurement — Two distinct interband transitions identified
  • Materials: White-light-continuum (WLC) probe; Femtosecond laser pulses
  • Methods: Pump-probe technique; Broadband probe; Reflection mode
  • Observations: Two high-energy electronic excitations at ≈1.8 eV and ≈2.4 eV; Red shifts of these excitations below T_DW
• Data analysis and modeling — Two DW gaps exist in La₃Ni₂O₇
  • Methods: Lorentzian fits; Rothwarf-Taylor model; BCS-type gap fitting
  • Observations: Two density-wave gaps: Δ(0)_HE₁=54 meV, Δ(0)_HE₂=67 meV; Slow decay components appear below T_DW
• Coherent phonon analysis — Phonon dynamics reveal electron-phonon coupling in DW state
  • Methods: Fast Fourier Transform (FFT); Anharmonic phonon model
  • Observations: Four coherent Raman-active phonon modes; Phonon softening with increasing temperature; Deviations at low T suggest electron-phonon coupling