SFB Colloquium

The pairing mechanism in iron-based superconductors is believed to be unconventional, i.e. not phonon-mediated. The achieved transition temperatures Tc in these superconductors are still significantly below those of some of the cuprates, with the exception of single layer FeSe films on SrTiO3 showing a Tc between 60 and 100 K, i.e. an order of magnitude larger than in bulk FeSe. This enormous increase of Tc demonstrates the potential of interface engineering for superconductivity. The underlying mechanism of Cooper pairing is, however, not understood. Both conventional and unconventional mechanisms have been discussed. We reveal how STM can directly measure the electron-boson coupling function in FeSe on SrTiO3. We shows that the excitation spectrum becomes fully gapped below Tc strongly supporting an electronic pairing mechanism. Finally, we will demonstrate, how the lateral resolution of STM can be used to resolve the contributions of different orbitals to the superconducting gap in multi-orbital superconductors. This approach is based on STM’s capability to resolve the local density of states (LDOS) with a combined high energy and sub unit-cell lateral resolution to directly image the orbital contributions to the superconducting gap on defect free FeSe crystals in real space.