Ultrafast Raman probe of the photoinduced superconducting to normal state transition in the cuprate Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$

We report an ultrafast Time-Resolved Raman scattering study of the out-of-equilibrium photoinduced dynamics across the superconducting to normal state phase transition of the cuprate Bi$_2$Sr$_2$CaCu$2$O${8+δ}$. Using the polarization-resolved momentum space selectivity of Raman scattering, we track the superconducting condensate destruction and recovery dynamics with sub-picoseconds time resolution in the anti-nodal region of the Fermi surface where the superconducting gap is maximum. Leveraging ultrafast Raman thermometry, we find a significant dichotomy between the superconducting condensate and the quasiparticle temperature dynamics near the anti-nodes, which cannot be framed in terms of a single effective electron temperature. The present work demonstrates the ability of Time-Resolved Raman scattering to selectively probe out-of-equilibrium pathways of different electronic sub-degrees of freedom during a photoinduced phase transition.

💡 Research Summary

In this work the authors present a comprehensive ultrafast time‑resolved Raman (TR‑Raman) investigation of the photo‑induced superconducting‑to‑normal phase transition in optimally doped Bi₂Sr₂CaCu₂O₈₊δ (Bi‑2212). By employing a pump‑probe scheme with a 150 fs, 1.2 eV pump and a 2.4 eV probe, they achieve sub‑picosecond temporal resolution (0.36–0.43 ps) while probing the B₁g Raman symmetry. This geometry selects the anti‑nodal regions of the Brillouin zone where the d‑wave gap reaches its maximum, allowing a direct view of the Cooper‑pair dynamics.

Equilibrium Raman spectra recorded from 4 K up to 113 K reveal two distinct spectral components: a sharp pair‑breaking peak centered around 500 cm⁻¹ (the Cooper‑pair signature) and a low‑energy Drude‑like background arising from thermally excited quasiparticles (QPs). The authors quantify these components by integrating the intensity between 370–725 cm⁻¹ (IΔ) for the pair‑breaking peak and between 130–200 cm⁻¹ (IQP) for the QP Drude part. IΔ follows a mean‑field BCS temperature dependence below Tc and becomes temperature‑independent above Tc, confirming its role as a proxy for the superconducting order parameter. IQP, in contrast, is essentially flat above Tc and grows with temperature below Tc, reflecting the increasing quasiparticle population.

When the sample is excited with increasing pump fluence (2–50 µJ cm⁻²) at 4 K, IΔ is dramatically suppressed, disappearing completely at a fluence of ≈30 µJ cm⁻². The critical fluence required to fully melt the superconducting condensate, FT, is determined to be 25 ± 5 µJ cm⁻², consistent with earlier optical and THz studies. Simultaneously, IQP rises, indicating that the electronic subsystem is heated far beyond what would be expected from a simple thermal equilibrium calculation.

Time‑resolved measurements at two representative fluences (F < FT and F ≈ FT) show that the pair‑breaking peak intensity drops within ~0.2 ps after the pump arrives, reaches a minimum around 1 ps, and then recovers on a picosecond timescale. The recovery can be described by a single exponential with a time constant τ ≈ 2.8 ps for both low and near‑critical fluences, while a higher fluence (F > FT) yields a slower component τ ≈ 4.8 ps after an initial plateau of ≈1.5 ps. The early‑time delay of the superconducting suppression relative to the transient reflectivity change (ΔR) measured in the normal state indicates that the condensate is not broken directly by the pump photons; instead, the destruction is mediated by bosonic excitations (phonons or magnons) generated during the rapid cooling of hot electrons. This interpretation aligns with Rothwarf‑Taylor‑type models that predict a fluence‑dependent increase of the destruction time due to the interplay between Cooper pairs and pair‑breaking bosons.

A crucial observation is the pronounced dichotomy between the dynamics of IΔ (the condensate) and IQP (the quasiparticle temperature). While IQP rises almost instantaneously, reflecting a rapid increase of the electronic temperature, IΔ remains finite for a longer period, showing that the superconducting order parameter does not follow the quasiparticle temperature on the same timescale. Consequently, the phase transition cannot be captured by a single effective electron temperature; instead, distinct sub‑degrees of freedom evolve non‑thermally and independently. By applying ultrafast Raman thermometry to the anti‑Stokes side of the spectrum, the authors extract the transient quasiparticle temperature and confirm that it exceeds the temperature inferred from the condensate dynamics.

Overall, the study demonstrates that ultrafast Raman spectroscopy, with its momentum‑space selectivity, provides a powerful window onto the anti‑nodal dynamics of high‑Tc cuprates. It reveals that the photo‑induced superconducting‑to‑normal transition proceeds via a non‑thermal, boson‑mediated pathway, and that the recovery of the condensate is governed by distinct timescales compared to the heating of quasiparticles. These findings complement ARPES studies that focus on the nodal region, highlighting the importance of anti‑nodal physics in the ultrafast control of superconductivity and offering new insights for designing light‑driven manipulation schemes in correlated electron materials.