Inequivalence of Single-Particle and Population Lifetimes in a Cuprate Superconductor

TL;DR

This study compares population and single-particle lifetimes in a cuprate superconductor, revealing significant differences even at low excitation, indicating additional scattering mechanisms beyond electron-phonon interactions.

Contribution

It provides the first direct comparison of population and single-particle lifetimes in a cuprate superconductor, highlighting the importance of non-electron-phonon scattering channels.

Findings

Population lifetimes deviate from single-particle lifetimes by one to two orders of magnitude.

Disparity persists even at low excitation and temperature, suggesting additional scattering mechanisms.

Electron-phonon scattering alone cannot explain the observed electron dynamics.

Abstract

We study optimally doped Bi-2212 ($T_\textrm{c}=96$~K) using femtosecond time- and angle-resolved photoelectron spectroscopy. Energy-resolved population lifetimes are extracted and compared with single-particle lifetimes measured by equilibrium photoemission. The population lifetimes deviate from the single-particle lifetimes in the low excitation limit by one to two orders of magnitude. Fundamental considerations of electron scattering unveil that these two lifetimes are in general distinct, yet for systems with only electron-phonon scattering they should converge in the low-temperature, low-fluence limit. The qualitative disparity in our data, even in this limit, suggests that scattering channels beyond electron-phonon interactions play a significant role in the electron dynamics of cuprate superconductors.