Imaging the Electron-Boson Coupling in Superconducting FeSe

TL;DR

This study uses scanning tunneling spectroscopy to investigate the electron-boson coupling in superconducting FeSe, revealing a bosonic mode below Tc and its relationship with the superconducting gap, challenging previous correlations observed in other high-Tc materials.

Contribution

It provides the first local spectroscopic evidence of a bosonic mode in FeSe and introduces a strong coupling model to explain its behavior, offering new insights into unconventional superconductivity.

Findings

Bosonic mode appears below Tc as a dip-hump feature at ~4.7 kBTc

Strained regions show simultaneous decreases in gap and mode energy

A local strong coupling model explains the observed correlations

Abstract

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 \Omega ~ 4.7 kBTc 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-Tc 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.