Tightly-bound Cooper pair, quasiparticle kinks and clues on the pairing potential in a high Tc FeAs Superconductor

TL;DR

This study uses photoemission to analyze electron dynamics and quasiparticle kinks in high Tc FeAs superconductors, revealing strong coupling behavior and short Cooper pair sizes, shedding light on the pairing mechanism.

Contribution

It provides detailed experimental evidence linking quasiparticle kinks to magnetic excitations and phonons, offering new insights into the pairing potential in high Tc FeAs superconductors.

Findings

Quasiparticle dispersion kinks at 15-50 meV match magnetic excitation energies.

Cooper pair wavefunction size is less than 20 Å, indicating unconventional pairing.

Strong coupling behavior with 2D/kBTc ratio of 6.

Abstract

We present a systematic photoemission study of the newly discovered high Tc superconductor class (Sr/Ba)1-xKxFe2As2. By utilizing a unique photon energy range and scattering geometry we resolve the details of the single particle dynamics of interacting electrons on the central Fermi surfaces of this series which shows overall strong coupling behavior (2D/kBTc = 6). Quasiparticle dispersion kinks are observed in a binding energy range of 15 to 50 meV which matches the magnetic excitation energy scales (parameterized by J1,J2). The size of the Cooper pair wavefunction is found to be less than 20A indicating a short in-plane scale uncharacteristic of a BCS-phonon scenario but suggestive of a phase factor in the global order parameter. The kink likely reflects contributions from the strongly frustrated fluctuating spin excitations and the soft phonons around 20-40 meV. Our results provide important clue to the nature of the pairing potential realized in these superconductors.