Persistence of Dirac Node near Antiferromagnetic-to-Superconducting Phase Boundary in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$

Since the ground state of iron-pnictides changes from an antiferromagnetic (AF) phase to a superconducting (SC) phase, the evolution of electronic structure has attracted much attention. However, systematic investigation has been hindered by the intricate multiple bands arising from the orbital degree of freedom of iron $3d$ states. Here we performed a polarization-dependent ARPES study of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ across the AF-SC phase boundary. The doping-dependence of ARPES spectra has shown that the Dirac node reported in the AF phase of BaFe$_2$As$_2$ persists in $x=0.04$ near the AF-SC phase boundary, and that it disappears in the SC phase of $x=0.05$. We parametrized the cone-like dispersion in $x=0.04$. The polarization-dependence of our ARPES spectra is consistent with the view that the Dirac node is protected by Berry phase arising from orbital degree of freedom under the inversion symmetry.