Abstract
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Scanning tunneling spectroscopy has been used to reveal signatures of a bosonic mode in the local
quasiparticle density of states of superconducting FeSe films. The mode appears below Tc as a “dip-hump”
feature at energy Ω ∼ 4.7kBTc beyond the superconducting gap Δ. Spectra on strained regions of the FeSe
films reveal simultaneous decreases in Δ and Ω. This contrasts with all previous reports on other high-Tc
superconductors, where Δ locally anticorrelates with Ω. A local strong coupling model is found to
reconcile the discrepancy well, and to provide a unified picture of the electron-boson coupling in
unconventional superconductors.