Energy-density-driven ultrafast electronic excitations in a cuprate superconductor

TL;DR

The paper investigates nonequilibrium dynamics in Bi2Sr2CaCu2O8+δ using ultrafast EUV pumping from a free-electron laser and 1.5 eV optical probing, comparing room-temperature and superconducting-phase responses to conventional optical pumping. It finds that the transient reflectivity dynamics are governed by the absorbed energy density, with similar fast (≈100–300 fs) and slow (≈1–5 ps) relaxation channels above Tc and a delayed quasiparticle-recombination component below Tc that can be suppressed at higher fluences. The results demonstrate a universal, energy-density–driven response that is largely independent of the specific microscopic excitation pathway, albeit with a higher energy requirement for EUV due to its shallower excitation volume. This work establishes FEL-based EUV pumping as a versatile platform for probing and controlling correlated electronic states, with promising prospects for future soft X-ray and attosecond studies of cuprate dynamics supported by advanced many-body theory and simulations.

Abstract

Controlling nonequilibrium dynamics in quantum materials requires ultrafast probes with spectral selectivity. We report femtosecond reflectivity measurements on the cuprate superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ using free-electron laser extreme-ultraviolet (23.5--177~eV) and near-infrared (1.5~eV) pump pulses. EUV pulses access deep electronic states, while NIR light excites valence-band transitions. Despite these distinct channels, both schemes produce nearly identical dynamics: above $T_c$, excitations relax through fast (100--300~fs) and slower (1--5~ps) channels; below $T_c$, a delayed component signals quasiparticle recombination and condensate recovery. We find that when electronic excitations are involved, the ultrafast response is governed mainly by absorbed energy rather than by the microscopic nature of the excitation. In contrast, bosonic driving in the THz or mid-infrared produces qualitatively different dynamics. By demonstrating that EUV excitation of a correlated superconductor yields macroscopic dynamics converging with those from optical pumping, this work defines a new experimental paradigm: FEL pulses at core-level energies provide a powerful means to probe and control nonequilibrium electronic states in quantum materials on their intrinsic femtosecond timescales. This establishes FEL-based EUV pumping as a new capability for ultrafast materials science, opening routes toward soft X-ray and attosecond studies of correlated dynamics.

Figures (4)

• Figure 1: Transient reflectivity dynamics at $T = 300$ K. (a) EUV-pump, optical-probe measurements for four different pump photon energies $E_{\mathrm{EUV}}$, each taken at an excitation energy density $I_{\mathrm{EUV}} \sim 200$--$300$ J/cm$^3$. The inset shows an expanded view of the first 2 ps of the dynamics. (b) Comparison between room-temperature dynamics induced by an EUV pump ($E_{\mathrm{EUV}} = 77$ eV) and by a near-IR pump ($E_{\mathrm{pump}} = 1.5$ eV, adapted from Ref. giannetti2009discontinuity).
• Figure 2: Fit parameters extracted from room-temperature transient reflectivity measurements as a function of excitation energy density $I_{\mathrm{EUV}}$ for all pump photon energies employed. (a,b) Fast and slow decay times, $\tau_1$ and $\tau_2$, respectively. (c,d,e) Amplitude parameters $a_1$, $a_2$, and $c$, each normalized by the excited volume fraction $v = V_{\mathrm{EUV}}/V_{\mathrm{probe}}$ to account for penetration-depth mismatch between pump and probe.
• Figure 3: Transient reflectivity dynamics in the superconducting state ($T < T_c$). (a) Fluence dependence of the $\Delta R/R$ response measured at $T = 30$ K with $E_{\mathrm{EUV}}=77$ eV for various excitation energy densities $I_{\mathrm{EUV}}$. High-fluence traces are rescaled by the normalization factors indicated in the legend to enable direct comparison of temporal profiles. For $I_{\mathrm{EUV}} < 100$ J/cm$^3$, an additional component, highlighted by the grey arrow, emerges with a build-up time of several hundred femtoseconds. This feature, absent in the high-fluence or high-temperature response, signals incomplete suppression of superconductivity. (b) Comparison of EUV-pump ($E_{\mathrm{EUV}} = 77$ eV) and near-IR-pump ($E_{\mathrm{pump}} = 1.5$ eV) dynamics. The FEL-pump trace was recorded at $T = 30$ K with $F = 220$ µ J/cm$^2$ ($I_{\mathrm{EUV}} = 60$ J/cm$^3$), while the optical-pump trace (adapted from Ref. giannetti2009discontinuity) was measured at $T = 20$ K with $F = 285$ µ J/cm$^2$ ($I_{\mathrm{opt}} = 11$ J/cm$^3$).
• Figure 4: Fit parameters extracted from low-temperature ($T < T_c$) transient reflectivity measurements as a function of excitation energy density $I_{\mathrm{EUV}}$. a–c) Decay times $\tau_1$, $\tau_2$, and $\tau_3$, respectively. d–g) Amplitude parameters $a_1$, $a_2$, $a_3$, and $c$, normalized to the excited-volume fraction at 77 eV ($v = V_{\mathrm{exc}}/V_{\mathrm{probe}} = 0.145$) for consistency with Fig. \ref{['fig: RT fit param']}. The $a_3$ term is included only for the two lowest measured excitation fluences, where the slow build-up component associated with superconducting condensate recovery is observed.