Ultrafast decoupling of quasiparticles and spin fluctuations in superconducting cuprates

TL;DR

This study uses ultrafast broadband and single-cycle spectroscopy to reveal that quasiparticles in high-Tc cuprates decouple from spin fluctuations within 90 femtoseconds, shedding light on their nonequilibrium dynamics.

Contribution

It demonstrates an ultrafast decoupling of quasiparticles from spin fluctuations in cuprates, a process previously unresolved in nonequilibrium superconductivity.

Findings

Electron-electron Umklapp scattering dominates within a few femtoseconds.

Rapid suppression of scattering rate associated with spin-fluctuation dressed carriers.

Decoupling of quasiparticles from spin fluctuations occurs on a 90 fs timescale.

Abstract

Understanding how quasiparticles are generated following a rapid quench of superconductivity in high-Tc cuprates is a key unresolved problem in nonequilibrium superconductivity. Here we resolve these processes in optimally doped YBCO [YBa2Cu3Oy(y=6.94, Tc=92 K)] using broadband (0.16 -4.1 eV, ca. 100 fs) and nearly single-cycle (6 fs) transient reflectivity spectroscopy. We show that within a few femtosecond, enhanced electron-electron Umklapp scattering dominates, signaling a transient modulation of long-range Coulomb interactions on the eV scale. This regime is followed by a rapid suppression of the scattering rate of the mid-infrared absorption associated with carriers dressed by spin fluctuations. We attribute this observation to an ultrafast decoupling of quasiparticles from the spin-fluctuation background, occurring on a 90 fs timescale set by the inverse optical gap. These findings reveal the correlated many-body dynamics underlying quasiparticle generation in cuprates and provide further clues for unconventional pairing mechanism.