Laser-Based ARPES Study on Superconducting Gap of Optimally-Doped (Ba0.6K0.4)Fe2As2 Superconductor

The determination of the superconducting gap and its symmetry are important in understanding the underlying superconductivity mechanism in the iron-based superconductors. However, the results on the superconducting gap in a prototypical iron-based superconductor (Ba$_{\mathrm{0.6}}$K$_{\mathrm{0.4}})$Fe$_{\mathrm{2}}$As$_{\mathrm{2}}$ remains controversial after eight years of extensive investigations. Here we report our high resolution laser-based angle-resolved photoemission (ARPES) measurements on the superconducting gap of an optimally-doped (Ba$_{\mathrm{0.6}}$K$_{\mathrm{0.4}})$Fe$_{\mathrm{2}}$As$_{\mathrm{2}}$ superconductor. With ultra-high energy and momentum resolutions from our new generation laser-based ARPES system, we have carried out precise measurements of the superconducting gap and its temperature and momentum dependences in (Ba$_{\mathrm{0.6}}$K$_{\mathrm{0.4}})$Fe$_{\mathrm{2}}$As$_{\mathrm{2}}$. Our results provide a unified picture for the superconducting gap in (Ba$_{\mathrm{0.6}}$K$_{\mathrm{0.4}})$Fe$_{\mathrm{2}}$As$_{\mathrm{2}}$ superconductor and solve a long-standing controversy. They also put an upper limit on the anisotropy of the superconducting gap in the iron-based superconductors.