Interplay of superconductivity and rattling phenomena in $\beta $-pyrochlore KOs$_{2}$O$_{6}$ studied by photoemission spectroscopy

The electronic structure near Fermi level of KOs$_{2}$O$_{6}$ is studied by a laser-excited photoemission spectroscopy. The superconducting(SC) gap clearly opens across the SC transition at 9.6 K, with the strong electron-phonon coupling value of 2$\Delta $(0)/k$_{B}$T$_{c} \quad \ge $ 4.56. Fitting analysis identifies clear anomalies at 7.5 K in the temperature dependences of the SC gap size and the quasiparticle relaxation lifetime. These anomalies and the fine spectral structures arising from phonons, suggest that the existence of the rattling behavior of K ions significantly affects the superconductivity in KOs$_{2}$O$_{6}$.