Imaging the Electron-Boson Coupling in Superconducting FeSe Films Using a Scanning Tunneling Microscope

Scanning tunneling spectroscopy has been used to reveal signatures of a bosonic mode in the local quasiparticle density of states of superconducting Fe Se films The mode appears below T-c as a dip-hump feature at energy Omega similar to 4.7k(B)T(c), beyond the superconducting gap Delta. Spectra on strained regions of the Fe Se films reveal simultaneous decreases in Delta and Omega. This contrasts with all previous reports on other high-T-c superconductors, where Delta locally anticorrelates 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.