Direct observation of the spin exciton in Andreev spectroscopy of iron-based superconductors

TL;DR

This study directly observes the spin exciton in iron-based superconductors through Andreev spectroscopy, confirming the $s_$ order parameter symmetry and supporting the spin-fluctuation pairing mechanism.

Contribution

It provides the first direct observation of the spin exciton in these materials, linking spectroscopic evidence to pairing symmetry and mechanism.

Findings

Observation of temperature-dependent spin exciton

Confirmation of extended $s_$-wave symmetry

Support for spin-fluctuation pairing mechanism

Abstract

Quasiparticle excitations provide viable information on the physics of unconventional superconductors. Higgs and Leggett modes are some of the classic examples. Another important bosonic excitation is the spin exciton originating from the sign-changing superconducting gap structure. Here we report a direct observation of the temperature-dependent spin exciton in the Andreev spectra of iron-based superconductors. Combined with the other experimental evidence, our observation confirms the extended $s$-wave ($s_\pm$) order parameter symmetry and indirectly proves the spin-fluctuation mechanism of Cooper pairing.