Pairing symmetry of iron-based superconductors revealed by ARPES

The recent discovery of superconductivity in iron-arsenic compounds with a transition temperature ($T_{c}$ ) as high as 56 K ended the monopoly of copper oxides in the family of high-$T_{c}$ superconductors. In this talk I will report our angle-resolved photoemission observation of the superconducting gap, including its momentum, temperature, and Fermi surface (FS) dependence in single crystals Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$ ($T_{c}$ = 37 K). We found two nodeless and nearly isotropic superconducting gaps around their respective FS sheets: a large gap ($\Delta \quad \sim $ 12 meV) on the two small hole-like and electron-like FS sheets, and a small gap ($\sim $ 6 meV) on the large hole-like FS. The isotropic pairing interactions are strongly orbital dependent, as the ratio 2$\Delta $/$k_{B}T_{c}$ switches from weak to strong coupling on different bands. In addition, we have observed a dispersion kink that is likely related to a spin mode. These results reveal the importance of inter-band interactions in the pairing mechanism, and support the anti-phase $s$-wave pairing symmetry in the Fe-based superconductors.