Fit-Free Optical Determination of Electronic Thermalization Time in Nematic Iron-Based Superconductors

We present a nematic response function model (NRFM) for fit-free direct extraction of the characteristic time of ultrafast electronic thermalization in iron-based superconductors, materials with electronic nematicity. By combining the NRFM for polarization-dependent pump–probe measurements of electronic nematic response with the two-temperature model (TTM) for sub-picosecond quasiparticle relaxation, we quantify the electronic thermalization timescales and their anisotropy. The nematic response function is modeled as the difference of normalized reflectivity signals, revealing a pronounced sub-picosecond extremum in signal evolution that directly yields the characteristic electronic thermalization time. This method demonstrates that the NRFM is consistent with TTM fits of transient optical response, yielding electronic thermalization time constants on the order of 110–230~fs for the FeSe${1-x}$Te$x$ and Ba(Fe${0.92}$Co${0.08}$)$_2$As$_2$ thin films. The proposed approach can be applied to any material that exhibits electronic nematicity, providing a powerful tool for direct mapping of the relaxation time in nematic materials, avoiding complex experimental data-fitting procedures.

💡 Research Summary

The authors introduce a “nematic response function model” (NRFM) that enables a fit‑free, direct extraction of the electronic thermalization time (τₑ) in iron‑based superconductors (FBS) exhibiting electronic nematicity. In a polarization‑resolved pump‑probe experiment, the transient reflectivity change ΔR/R is recorded for two orthogonal linear polarizations (parallel and perpendicular to the nematic axis). By normalizing each trace and taking their difference η(t)=ΔR∥/R−ΔR⊥/R, the authors obtain a function that, at early times (t < τₑ‑ph), is well described by the difference of two exponential decays with characteristic times τ∥ and τ⊥. The function possesses a single extremum (a minimum for τ⊥ > τ∥). The temporal position of this minimum, t_min, is shown analytically to be essentially the average electronic thermalization time τ_avg = (τ∥ + τ⊥)/2, with a small correction that depends on the instrument response function (IRF) width. By correcting for the finite IRF (Gaussian with full‑width‑half‑maximum τ_IRF≈50 fs), the authors derive a compact relation τ_avg≈t_min − τ_IRF²/(κ²·t_min) (κ = 2√(2 ln 2)).

Crucially, the amplitude of the minimum, η_min, encodes the difference Δτ = τ⊥ − τ∥. Assuming equal amplitudes for the two polarizations (r = A∥/A⊥ ≈ 1, enforced by post‑processing normalization), the authors obtain Δτ≈−e·η_min·τ_avg·exp