Quantum vortex core and missing pseudogap in the multi-band BCS-BEC-crossover superconductor FeSe*

The iron-based superconductor FeSe is considered to be in the BCS-BEC crossover regime where the superconducting gap is comparable to the Fermi energy. We performed spectroscopic-imaging scanning tunneling microscopy to search for spectroscopic signatures of the BCS-BEC crossover. The large superconducting-gap to Fermi-energy ratio may yield well separated discrete bound states in the vortex core, as well as a pseudogap above the superconducting transition temperature due to preformed Cooper pairs. We found a signature of the discrete vortex bound states but could not detect the pseudogap. We argue that these conflicting observations can be resolved if multi-band nature is taken into account.