Ultrafast decoupling of quasiparticles and spin fluctuations in superconducting cuprates

Yuto Taniguchi; Ryo Kato; Tatsuya Amano; Hirotake Itoh; Yohei Kawakam; Yuto Nakamura; Hideo Kishida; Christian Bernhard; Jure Demsar; Takahiko Sasaki; Terukazu Nishizaki; Kenji Yonemitsu; Shinichiro Iwai

arXiv · 2605.14238

Ultrafast decoupling of quasiparticles and spin fluctuations in superconducting cuprates

Yuto Taniguchi·Ryo Kato·Tatsuya Amano·Hirotake Itoh·Yohei Kawakam·Yuto Nakamura·Hideo Kishida·Christian Bernhard·Jure Demsar·Takahiko Sasaki·Terukazu Nishizaki·Kenji Yonemitsu·Shinichiro Iwai

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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.

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Yuto Taniguchi, Ryo Kato, Tatsuya Amano, Hirotake Itoh, Yohei Kawakam, Yuto Nakamura, Hideo Kishida, Christian Bernhard, Jure Demsar, Takahiko Sasaki, Terukazu Nishizaki, Kenji Yonemitsu, Shinichiro Iwai. 2026-05-14. Ultrafast decoupling of quasiparticles and spin fluctuations in superconducting cuprates. https://arxiv.org/abs/2605.14238

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