Direct observation of superconducting gap in YB₆ by ultrahigh-resolution photoemission spectroscopy

We have performed ultrahigh-resolution photoemission spectroscopy (PES) on superconducting hexaboride YB₆ (Tc=7.1 K) to investigate the electronic structure in the vicinity of the Fermi level (EF). We have succeeded in directly observing the superconducting gap together with a sharp coherent peak in the superconducting state (5K). From the numerical simulation of the PES spectra by using isotropic Dynes function, we have estimated the size of the superconducting gap to be 0.9 meV, which corresponds to 2Δ (0)/kBTc=3.8. We found a dip structure around 10 meV with respect to EF in the PES spectrum in the superconducting state. This structure is attributed to the strong coupling feature of electrons with the 10 meV acoustic phonon. These results indicate that YB₆ is categorized into phonon-mediated BCS superconductor in the strong or intermediate coupling regime.