Electronic Nematicity Revealed by Polarized Ultrafast Spectroscopy in Bilayer La₃Ni₂O₇

Summary

Using polarized ultrafast pump-probe spectroscopy, the researchers comparatively investigated the normal-state electronic dynamics of bilayer La₃Ni₂O₇ and trilayer La₄Ni₃O₁₀ single crystals under ambient pressure. Both materials exhibit a density-wave transition accompanied by the opening of a quasiparticle relaxation bottleneck, yet their electronic responses display markedly different symmetries: trilayer La₄Ni₃O₁₀ remains optically isotropic across the entire temperature range, whereas bilayer La₃Ni₂O₇ shows clear twofold (C₂) rotational symmetry breaking—i.e., electronic nematicity—at low temperatures. This nematicity manifests in the anisotropy of slow quasiparticle relaxation dynamics and effective gap scale, and below 115 K it competes with a secondary isotropic order, leading to a non-monotonic temperature dependence of the nematic signal. This work reveals the presence of electronic nematic fluctuations in bilayer nickelates, which are absent in the trilayer system, suggesting a close relationship between electronic nematicity and high-pressure superconducting pairing in La₃Ni₂O₇, thereby providing key insights into the microscopic mechanism of this class of nickel-based superconductors.

Materials

• La₃Ni₂O₇
• La₄Ni₃O₁₀

Methods

• Polarized ultrafast pump-probe spectroscopy
• Rothwarf-Taylor model fitting
• polarized ultrafast pump-probe spectroscopy

Keywords

• electronic nematicity
• density wave transition
• quasiparticle relaxation bottleneck
• twofold rotational symmetry breaking
• c2 symmetry
• c2 anisotropy

Highlights

• The presence of macroscopic electronic anisotropy in the bilayer system, and its absence in the trilayer system, suggests an intimate relation between electronic nematic fluctuations and superconducting pairing in La₃Ni₂O₇.
• This nematicity manifests as a significant anisotropy in the slow quasiparticle relaxation dynamics and effective gap scales.
• Electronic nematicity revealed by polarized ultrafast spectroscopy in bilayer La₃Ni₂O₇.
• The twofold anisotropy and competition with isotropic order provide deeper exploration for superconducting pairing.

Conclusions

• La₃Ni₂O₇ exhibits a pronounced rotational-symmetry-breaking electronic response with twofold anisotropy, indicating electronic nematicity.
• The non-monotonic temperature dependence of the nematic signal indicates its competition with a secondary, isotropic density-wave-like order below 115 K.
• La₄Ni₃O₁₀ maintains a robustly isotropic optical response over the entire temperature range, showing no nematicity.
• La₄Ni₃O₁₀ maintains an isotropic optical response across the entire temperature range.
• La₃Ni₂O₇ exhibits a pronounced twofold (C₂) anisotropy in its low-temperature electronic dynamics.
• This electronic nematicity competes with a secondary isotropic order emerging below 115 K.
• The presence of macroscopic electronic anisotropy in the bilayer system and its absence in the trilayer system suggests an intimate relation between electronic nematic fluctuations and superconducting pairing in La₃Ni₂O₇.

Main claims

• Bilayer La₃Ni₂O₇ exhibits electronic nematicity with twofold (C₂) rotational symmetry breaking at low temperatures, while trilayer La₄Ni₃O₁₀ remains isotropic.
  • Evidence: Abstract: 'La₃Ni₂O₇ shows clear twofold (C₂) rotational symmetry breaking—i.e., electronic nematicity—at low temperatures'
• The nematicity competes with a secondary isotropic order emerging below 115 K, leading to a non-monotonic temperature dependence of the nematic signal.
  • Evidence: Abstract: 'below 115 K it competes with a secondary isotropic order, leading to a non-monotonic temperature dependence of the nematic signal'
• La₃Ni₂O₇ exhibits a pronounced twofold (C₂) anisotropy in its low-temperature electronic dynamics, indicative of electronic nematicity.
  • Evidence: Normalized transient reflectivity (ΔR/R)_Norm shows clear deviation between φ=0° and φ=90° below T_SDW.,Polar plots of τ_s show isotropic at 160 K, two-fold at 115 K and 70 K.,Difference plot τ_ani = τ_s0 - τ_s90 peaks near 115 K.
• The electronic nematicity in La₃Ni₂O₇ competes with a secondary isotropic order emerging below 115 K.
  • Evidence: τ_ani shows non-monotonic behavior: increases below T_SDW, peaks near 115 K, then decreases upon further cooling.,Optical measurements report opening of an energy gap below 115 K, possibly associated with CDW order.
• La₄Ni₃O₁₀ maintains an isotropic optical response across the entire temperature range, showing no rotational symmetry breaking.
  • Evidence: (ΔR/R)_Norm curves for φ=0° and φ=90° completely overlap at all temperatures.,Polar plots of τ_s remain circular at all measured temperatures (130 K, 115 K, 70 K).
• The presence of electronic nematicity in bilayer La₃Ni₂O₇ and its absence in trilayer La₄Ni₃O₁₀ suggests an intimate relation between nematic fluctuations and superconducting pairing in La₃Ni₂O₇.
  • Evidence: Nematicity in iron-based and cuprate superconductors is linked to enhanced T_c near a nematic quantum critical point.,The observed anisotropy is spin-driven (SDW gap) and may provide additional attractive interaction for pairing.

Workflow

• Sample preparation
  • Materials: La₃Ni₂O₇ and La₄Ni₃O₁₀ single crystals
  • Methods: Vertical optical image floating-zone method; Cleavage of (001) planes
  • Observations: High-quality single crystals with well-defined surfaces
• Ultrafast pump-probe spectroscopy
  • Materials: Single crystals
  • Methods: Polarized ultrafast pump-probe measurement; Rotation of half-wave plate and polarizer
  • Observations: Normalized transient reflectivity along 0° and 90°
• Data analysis — Bilayer exhibits electronic nematicity that competes with isotropic order below 115K
  • Materials: Transient reflectivity data
  • Methods: Two-exponential decay model; Rothwarf-Taylor model
  • Observations: La₃Ni₂O₇ shows C₂ rotational symmetry breaking below SDW transition; La₄Ni₃O₁₀ remains isotropic
• sample_preparation
  • Materials: La₃Ni₂O₇ single crystals; La₄Ni₃O₁₀ single crystals
  • Methods: vertical optical image floating-zone method
• measurement
  • Materials: single crystals of La₃Ni₂O₇ and La₄Ni₃O₁₀
  • Methods: polarized ultrafast pump-probe spectroscopy
  • Observations: normalized transient reflectivity (ΔR/R)_Norm measured as function of delay time; angular dependence of (ΔR/R)_Norm and relaxation times
• analysis
  • Materials: measured ΔR/R data
  • Methods: two-exponential decay model fitting; Rothwarf-Taylor (RT) model fitting for gap extraction
  • Observations: τ_f and τ_s extracted for both polarizations; temperature dependence of τ_f and τ_s; difference plots (ΔR/R)_ani and τ_ani = τ_s0 - τ_s90
• interpretation — La₃Ni₂O₇ exhibits electronic nematicity (C₂ rotational symmetry breaking) in its low-temperature electronic dynamics, while La₄Ni₃O₁₀ remains isotropic.
  • Methods: comparison of symmetry properties between La₃Ni₂O₇ and La₄Ni₃O₁₀; analysis of rotational symmetry using polar plots of τ_s
  • Observations: La₄Ni₃O₁₀: isotropic response across all temperatures; La₃Ni₂O₇: isotropic at high T, pronounced two-fold (C₂) anisotropy below T_SDW ≈140K; non-monotonic τ_ani peaking near 115 K, suppressed by secondary isotropic order below 115K