Imaging the Electron-Boson Coupling in Superconducting FeSe

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 $T_c$ as a `dip-hump' feature at energy $\Omega\sim 4.7 k_B T_c$ beyond the superconducting gap $\Delta$. Spectra on strained regions of the FeSe films reveal simultaneous decreases in $\Delta$ and $\Omega$. This contrasts with all previous reports on other high-$T_c$ superconductors, where $\Delta$ locally anti-correlates with $\Omega$. 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.