Nonthermal melting and density wave instability coupled to the lattice in La₄Ni₃O₁₀

Summary

Using ultrafast optical spectroscopy on La₄Ni₃O₁₀ single crystals, researchers observed an abrupt change in quasiparticle relaxation dynamics at the density-wave transition temperature of approximately 136 K, revealing the opening of a strongly coupled energy gap of about 52 meV. Multiple coherent phonon modes, including Ag modes near 3.88, 5.28, and 2.09 THz, exhibited mode-selective anomalies across the transition, with the renormalization behavior of the 3.88 THz phonon in particular shifting from conventional anharmonic decay at high temperatures to pronounced hardening at low temperatures, indicating strong coupling between the density-wave instability and lattice degrees of freedom and suggesting that electron–phonon interactions likely play a critical role. Under high excitation fluence, the density wave is suppressed non-thermally, yielding a temperature–fluence phase diagram that resembles the pressure-tuning behavior, though the gap remains relatively stable, leading to an increased coupling ratio. These findings establish the density wave in La₄Ni₃O₁₀ as a lattice-entangled instability involving multiorbital physics and confirm that ultrafast photoexcitation can serve as a non-equilibrium control parameter to effectively suppress density-wave order in nickelates.

Materials

• La₄Ni₃O₁₀

Methods

• Pump-probe ultrafast optical spectroscopy
• Rothwarf-Taylor analysis

Keywords

• density wave instability
• nonthermal melting
• electron phonon coupling
• lattice entangled instability
• strong coupling gap

Highlights

• The 3.88 THz phonon mode exhibits anomalous hardening below T_DW with a power-law exponent switching from 2 to 5.2, indicating strong coupling to the density wave.
• At high fluence, the density wave is nonthermally suppressed while the energy gap remains approximately constant, leading to an enhanced coupling ratio 2Δ/k_B T_DW ≈ 6.6.
• A phonon mode at 5.62 THz emerges only below T_DW, possibly due to zone folding from symmetry lowering.
• The density wave in La₄Ni₃O₁₀ is a lattice-entangled instability involving multiple phonon modes, distinguishing it from bilayer La₃Ni₂O₇.

Conclusions

• Ultrafast optical spectroscopy reveals a strong-coupling density wave gap of ≈52 meV in La₄Ni₃O₁₀ single crystals at T_DW ≈ 136 K.
• Multiple coherent phonons exhibit mode-selective anomalies, indicating strong electron-phonon coupling and multiorbital physics.
• The density wave is nonthermally suppressed at high excitation fluences, producing a temperature-fluence phase diagram analogous to pressure tuning.
• These results establish the density wave as a lattice-entangled instability and highlight ultrafast optical excitation as a nonequilibrium tuning parameter for nickelates.

Main claims

• Quasiparticle relaxation dynamics reveal the opening of a strong-coupling 52 meV density-wave gap at T_DW ≈136 K.
  • Evidence: Slow quasiparticle component lifetime τ_slow shows critical slowing down at T_DW; Rothwarf-Taylor analysis yields a zero-temperature gap 2Δ(0) ≈52 meV; ratio 2Δ/k_BT_DW ≈ 4.4, exceeding the BCS weak-coupling value of 3.5.
• A phonon mode at 3.88 THz undergoes anomalous renormalization strongly coupled to the DW instability, indicating strong electron-phonon coupling and multiorbital involvement.
  • Evidence: Frequency of 3.88 THz mode follows standard anharmonic decay (exponent n=2) above T_DW, but exhibits pronounced hardening with n≈5.2 below T_DW; eigenvector involves apical oxygen motion, Ni-O plane buckling, and La displacements; other phonons show contrasting behaviors (softening of 3.55 THz and 2.09 THz modes), highlighting mode-selective coupling.
• Intense optical excitation suppresses the density wave nonthermally, producing a temperature-fluence phase diagram analogous to pressure tuning, while the gap remains relatively stable.
  • Evidence: At high fluence (130 μJ/cm²), T_DW is suppressed to 95 K, but the extracted gap remains ≈55 meV, enhancing 2Δ/k_BT_DW to 6.6; critical fluence for DW melting increases with decreasing temperature; steady-state laser heating accounts for only an 8 K temperature rise, far less than the 40 K suppression, confirming a nonthermal mechanism.

Workflow

• sample_preparation — High-quality single crystals of La₄Ni₃O₁₀ were obtained via optical floating-zone growth under oxygen pressure.
  • Materials: La₄Ni₃O₁₀ single crystals; optical floating-zone furnace; O₂ atmosphere (18-22 bar)
  • Methods: high-pressure optical floating-zone growth
  • Observations: high-quality single crystals grown
• ultrafast_optical_spectroscopy — Ultrafast pump-probe spectroscopy captures sub-picosecond quasiparticle dynamics and coherent lattice vibrations across the density-wave transition.
  • Materials: pump-probe setup; 1-MHz Yb-fiber femtosecond oscillator; optical parametric amplifier; 800 nm (1.55 eV) pulses
  • Methods: transient differential reflectivity measurements; temperature-dependent scans at weak and high fluences
  • Observations: biexponential quasiparticle relaxation with fast and slow components; coherent phonon oscillations observed across T_DW
• data_analysis — A strong-coupling density-wave gap opens at T_DW; multiple phonons show mode-selective electron-phonon coupling; nonthermal melting of DW order is evidenced by the fluence-dependent phase diagram.
  • Materials: transient reflectivity traces; fast Fourier transform spectra
  • Methods: biexponential fitting of QP relaxation; Rothwarf-Taylor model for gap extraction; phonon frequency tracking and power-law fitting; phase diagram construction
  • Observations: DW gap 2Δ(0) ≈52 meV, ratio 2Δ/k_BT_DW ≈4.4 (strong coupling); phonon modes at 3.88, 5.28, 2.09 THz exhibit mode-selective anomalies; 3.88 THz mode hardens below T_DW (exponent n≈5.2 vs n=2 above); high fluence suppresses T_DW while gap remains ≈55 meV, ratio increases to 6.6; multiple emergent phonon modes; 2.09 THz softens below T_DW; phonon at 5.62 THz appears only below T_DW
• interpretation — The density wave in La₄Ni₃O₁₀ is a lattice-entangled instability with strong electron-phonon coupling and multiorbital character; ultrafast optical excitation provides a nonequilibrium pathway to suppress and tune density-wave order in nickelates.
  • Materials: temperature-fluence phase diagram; comparison with pressure-tuned behavior
  • Methods: comparison with literature on pressure effects; analysis of electron-phonon coupling via phonon eigenvectors and isotope effect
  • Observations: nonthermal suppression of T_DW without a proportional reduction of the gap; enhanced 2Δ/k_BT_DW under high fluence; lattice-entangled instability involving multiple phonon modes and multiorbital physics