Ultrafast Gap Dynamics and Electronic Interactions in a Photoexcited Cuprate Superconductor

Abstract

We perform time- and angle-resolved photoemission spectroscopy (trARPES) on optimally doped Bi2Sr2CaCu2O8+δ (BSCCO-2212) using sufficient energy resolution (9 meV) to resolve the k -dependent near-nodal gap structure on time scales where the concept of an electronic pseudotemperature is a useful quantity, i.e., after electronic thermalization has occurred. We study the ultrafast evolution of this gap structure, uncovering a very rich landscape of decay rates as a function of angle, temperature, and energy. We explicitly focus on the quasiparticle states at the gap edge as well as on the spectral weight inside the gap that “fills” the gap—understood as an interaction, or self-energy effect—and we also make high resolution measurements of the nodal states, enabling a direct and accurate measurement of the electronic temperature (or pseudotemperature) of the electrons in the system. Rather than the standard method of interpreting these results using individual quasiparticle scattering rates that vary significantly as a function of angle, temperature, and energy, we show that the entire landscape of relaxations can be understood by modeling the system as following a nonequilibrium, electronic pseudotemperature that controls all electrons in the zone. Furthermore, this model has zero free parameters, as we obtain the crucial information of the SC gap Δ and the gap-filling strength ΓTDoS by connecting to static ARPES measurements. The quantitative and qualitative agreement between data and model suggests that the critical parameters and interactions of the system, including the pairing interactions, follow parametrically from the electronic pseudotemperature. We expect that this concept will be relevant for understanding the ultrafast response of a great variety of electronic materials, even though the electronic pseudotemperature may not be directly measurable.

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Plain Language Summary

High-temperature superconductivity, in which electricity flows through a substance without resistance at temperatures well above absolute zero, has inspired decades of experimental advances in the quest to understand the exotic physics. One of the most valuable tools to emerge in recent years is “time- and angle-resolved photoemission spectroscopy” (trARPES), which uses ultrashort pulses of laser light to reveal how electrons move and interact in a solid. While trARPES investigations on superconductors have revealed rich behavior, physicists disagree on the interpretation, leaving open several questions. Specifically, researchers would like to understand how photoexcitation (in which an electron gains energy by absorbing a photon) affects a superconductor and what that can reveal about superconductivity itself. Using trARPES to study a type of superconductor, we find that the photoexcited electrons rapidly redistribute their energy among the whole population, i.e., rapidly thermalize, and the rich behavior observed is a consequence of the interplay between these “hot” electrons and the underlying superconductivity.

Our work uses significantly improved energy resolution over previous research, which allows for a clearer picture of the superconducting physics. Furthermore, by drawing on results from previous high-resolution ARPES experiments, we are able to show how the quantities important to superconductivity in a photoexcited system, such as electron pairing and self-energies, compare and connect to their equilibrium values. Overall, our investigation ties together previous results under a straightforward interpretation and gives a clear picture of how superconductivity responds to photoexcitation.

Under sufficiently intense photoexcitation, or at sufficiently short times, we expect this thermalized electron approximation to break down. Therefore, our results provide a natural reference point for future studies exploring photoexcited superconductors in these new regimes.