Anisotropic Superconducting Gaps and Boson Mode in FeSe 1−x Sx Single Crystals

Scanning tunneling spectroscopy has been used to investigate the superconducting gaps of FeSe 1−x S x single crystals and to reveal signatures of a bosonic mode in the quasiparticle density of states. We find that both superconducting gaps residing on different pockets of the Fermi surface are anisotropic. Moreover, the bosonic mode appears in the quasiparticle density of states as a redistribution of states at energy Ω/e, measured with respect to the superconducting gap. The energy of the boson mode Ω is found to scale with the superconducting gap, and it can be estimated to be in the range 2.6 ÷ 3.8 meV in agreement with a recent observation of a resonance spin excitation in neutron scattering. This suggests that quasiparticle interactions with this mode are important for superconductivity.