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

摘要

该研究利用时间分辨光学光谱技术，在环境压力下10 K至室温范围内探索了双层镍氧化物La₃Ni₂O₇中高能电子激发的超快动力学。研究识别出源于不同带间跃迁的两个高能电子激发，能量分别位于约1.8 eV和2.4 eV，并揭示它们具有不同的密度波（DW）能隙，分别为约54 meV和67 meV。这两个激发态的弛豫动力学可由Rothwarf-Taylor模型很好地描述。此外，实验观测到四个相干拉曼活性声子模式，这些模式与不同的电子激发表现出不同的耦合强度。升温引起的声子软化在约100 K至室温之间可用包含热膨胀和非谐声子-声子耦合的半定量模型解释；而在低温下，测量到的声子频率与模型拟合的偏差暗示了电子-声子耦合的额外贡献。该工作直接证明了该材料中复杂的能隙结构和声子动力学，为理解其密度波机制及多体效应提供了关键见解。

材料

• La₃Ni₂O₇ bulk crystals

方法

• time-resolved optical spectroscopy
• Rothwarf-Taylor model

关键词

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

亮点

• 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.

结论

• 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.

主要论断

• Two distinct density-wave gaps of ≈54 meV and ≈67 meV exist in La₃Ni₂O₇
  • 证据: 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
  • 证据: 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

研究流程

• Sample preparation
  • 材料: La₃Ni₂O₇ bulk crystal
  • 方法: Growth; Cryostat mounting
• Time-resolved optical spectroscopy measurement — Two distinct interband transitions identified
  • 材料: White-light-continuum (WLC) probe; Femtosecond laser pulses
  • 方法: Pump-probe technique; Broadband probe; Reflection mode
  • 观察: 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
  • 方法: Lorentzian fits; Rothwarf-Taylor model; BCS-type gap fitting
  • 观察: 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
  • 方法: Fast Fourier Transform (FFT); Anharmonic phonon model
  • 观察: Four coherent Raman-active phonon modes; Phonon softening with increasing temperature; Deviations at low T suggest electron-phonon coupling