Sub-Picosecond Electronic Switch Could Boost Future Superconductor Technology

Researchers are increasingly focused on understanding the ultrafast dynamics governing electronic thermalization in complex materials. Alexander Bartenev, Roman Kolodka, and Adrian Rua-Melendez, from the University of Puerto Rico, alongside Jason Kawasaki, Chang-Beom Eom, and Armando Rua, et al., from the University of Wisconsin, Madison, have developed a novel method for determining electronic thermalization time in nematic iron-based superconductors without relying on complex fitting procedures. Their work introduces a nematic response function model (NRFM) which, when combined with a two-temperature model, allows for direct extraction of thermalization timescales and their anisotropy from pump-probe measurements. This approach yields characteristic times in the 110, 230 femtosecond range for FeSeTe and Ba(FeCo)₂As₂ thin films and offers a broadly applicable technique for characterising the relaxation dynamics of any material exhibiting electronic nematicity.

This breakthrough circumvents the need for complex data fitting procedures typically associated with characterizing these timescales.

The work centres on a nematic response function model, or NRFM, which extracts the characteristic thermalization time from polarization-dependent pump, probe measurements of the electronic nematic response. By combining the NRFM with the established two-temperature model, or TTM, for sub-picosecond quasiparticle relaxation, scientists can now quantify these crucial electronic timescales and their directional dependence with unprecedented accuracy.
The core of this innovation lies in modelling the nematic response function as the difference between normalized reflectivity signals. This approach reveals a distinct sub-picosecond extremum in the signal evolution, directly corresponding to the characteristic electronic thermalization time. Demonstrating consistency with TTM fits of transient optical response, the NRFM yields electronic thermalization time constants ranging from 110 to 230 femtoseconds for FeSeTe and Ba(Fe0.92Co0.08)2As2 thin films.

This method provides a powerful tool for mapping relaxation times in nematic materials, offering a significant advancement over existing techniques. The study establishes a direct link between the position of an extremum in the nematic response function and the average electronic relaxation time of the material.

Specifically, the research demonstrates that for materials with small differences in relaxation times along orthogonal directions, the temporal position of the signal’s minimum corresponds to the average of the relaxation times. This allows for a ‘fit-free’ determination of the thermalization time, simplifying analysis and enhancing the reliability of measurements. The proposed approach is broadly applicable to any material exhibiting electronic nematicity, opening new avenues for investigating the fundamental physics of these complex materials and their potential for technological applications.

Determining ultrafast electronic thermalisation timescales via polarisation-dependent reflectivity measurements

A nematic response function model (NRFM) serves as the foundation for quantifying ultrafast electronic thermalization in iron-based materials exhibiting electronic nematicity. The research combines this NRFM, applied to polarization-dependent pump, probe measurements, with a two-temperature model (TTM) to determine electronic thermalization timescales and their anisotropy.

The nematic response function is defined as the difference of normalized reflectivity signals, and a pronounced sub-picosecond extremum within this signal evolution directly reveals the characteristic electronic thermalization time. Polarization-dependent pump, probe spectroscopy was employed to induce and monitor transient changes in reflectivity, driven by the excitation of quasiparticles and subsequent thermalization processes.

The resulting time-dependent reflectivity changes, denoted as ∆R/R, are attributed to hot-electron thermalization followed by electron, phonon scattering. The intrinsic material response is then modelled as a sum of exponential decays, incorporating amplitudes for both electron-electron and electron-phonon relaxation mechanisms, alongside their respective characteristic timescales, τe and τe−ph.

To account for experimental limitations, the recorded signal undergoes deconvolution with the instrument response function (IRF), effectively isolating the intrinsic material response. This methodology yields electronic thermalization time constants ranging from 110, 230 fs for FeSe1−xTex and Ba(Fe0.92Co0.08)2As2 thin films.

The study extends the analysis to both infinitesimally short and finite-width optical pulses, refining the precision of the extracted time constants. By comparing results obtained via the NRFM with those derived from TTM fits of the transient optical response, the work confirms consistency and provides insights into the anisotropy of quasiparticle relaxation processes.

Direct extraction of electronic thermalization times via nematic response function analysis

Electronic thermalization time constants between 110 and 230 femtoseconds were determined for FeSeTe and Ba(FeCo)₂As₂ thin films using a nematic response function model (NRFM). This work presents a method for fit-free direct extraction of characteristic thermalization times in materials exhibiting electronic nematicity.

The NRFM, combined with a two-temperature model (TTM), quantifies the timescales of electronic thermalization and their anisotropy through analysis of polarization-dependent pump-probe measurements. The nematic response function is defined as the difference of normalized reflectivity signals, revealing a pronounced extremum at approximately 150 femtoseconds that directly corresponds to the characteristic electronic thermalization time.

Determination of the average time constant, τavg, relies on the temporal position of this extremum, tmin, rather than complex fitting procedures. Normalization of the channels to unity does not shift tmin, ensuring reliability of the extracted value, which is controlled by signal-to-noise ratio, baseline drift, zero-time determination, and instrument response function width.

For FeSe₀.₈Te₀.₂ epitaxial film at 8 Kelvin, a sharp initial drop in transient reflectivity was attributed to hot-electron thermalization, followed by energy redistribution between electrons and the lattice. The resulting nematic response function, η(t), exhibited a minimum at tmin ≈150 fs. TTM fitting yielded electronic thermalization time constants closely matching those derived from the NRFM, validating the approach and demonstrating that both methods capture the same physical timescale.

Differences, ∆τ, between the models remained consistent across a range of pump fluences. Similar subpicosecond transient dynamics were observed in FeSe thin film at 3 Kelvin and Ba(Fe₀.₉₂Co₀.₀₈)₂As₂ at 8 Kelvin. For Ba(Fe₀.₉₂Co₀.₀₈)₂As₂, the electronic thermalization time constant, τe, showed minimal fluence dependence, with both models yielding comparable values differing by approximately one pulse width. The mean values τavg obtained from NRFM consistently aligned with the τe values from TTM, remaining within the experimental time resolution and confirming the accuracy of the NRFM approximation.

Electronic thermalization timescales in iron superconductors determined via polarised reflectivity

A new methodology based on polarization-resolved transient reflectivity measurements allows for the direct extraction of ultrafast electronic thermalization times in iron-based superconductors exhibiting electronic nematicity. By employing a nematic response function model alongside the two-temperature model, researchers have quantified the timescales of electronic thermalization and observed their anisotropy.

This approach identifies a sub-picosecond extremum in the signal evolution, directly revealing the characteristic electronic thermalization time, and yields values in the range of 110, 230 femtoseconds for FeSeTe and Ba(FeCo)₂As₂ thin films. The consistency between time constants derived from the nematic response function model and independent two-temperature model analysis validates the robustness of this technique.

A key advantage of this method is its extraction of the electronic thermalization time from a stable time marker, reducing sensitivity to fitting parameter choices compared to conventional global fitting approaches. This facilitates high-throughput analysis of relaxation trends influenced by factors such as temperature, doping, or fluence.

The proposed model extends beyond iron-based superconductors, offering a quantitative probe of anisotropic electron dynamics applicable to a wider range of correlated materials with electronic nematicity, and enabling detailed investigation of photoinduced nematic state dynamics. Limitations acknowledged by the authors include the need for corrections to account for the finite time resolution of the experimental setup, as detailed in the supplementary material. Future research may focus on applying this methodology to a broader range of materials exhibiting electronic nematicity to further establish its versatility and refine understanding of anisotropic electron dynamics in correlated materials.