INSP - Sorbonne Université - 4 place Jussieu - 75005 Paris - Barre 22-32, 2e étage, salle 201
Wulf Wulfhekel - Physikalisches Institut, KIT, Karlsruhe, Germany
Abstract
Superconductivity is one of the most exciting phenomena in correlated electron physics. While conventional superconductors with phonon mediated Cooper pairing seem well understood, the pairing mechanism in unconventional superconductors is still unclear. Here we report a STM-based method to directly measure the electron-boson coupling fundamentally based on the optical theorem. The inelastic scattering cross section of low-energy electrons is ultimately linked to the Eliashberg function describing the coupling. We illustrate the capabilities of the technique on conventional superconductors and show pathways to largely boost their electron-phonon coupling. Further, we investigate monolayer FeSe films on STO and exclude strong electron-phonon coupling except for places near domain boundaries. Instead, the bosonic excitation spectrum is shown to be fully gapped below Tc in agreement with an electronic pairing mechanism. The appearance of a resonance mode near 1.3 Δ also indicates a sign changing order parameter. Finally, we present a new technique to investigate the orbital character of multi-band unconventional superconductivity using spectroscopy at mK temperatures.