Experimental investigation of the suppressed superconducting gap and double-resonance mode in Ba1−xKxFe2As2

$\mathrm{B}{\mathrm{a}}_{1\ensuremath{-}x}{\mathrm{K}}_x\mathrm{F}{\mathrm{e}}₂\mathrm{A}{\mathrm{s}}₂$ has an exotic physical property, i.e., the ${C}₂$ spin-density-wave phase suddenly changes to the ${C}₄$ magnetic phase, and superconductivity is suppressed within a narrow composition range around $x=0.25$. We have investigated the doping dependence of the electronic structure of $\mathrm{B}{\mathrm{a}}_{1\ensuremath{-}x}{\mathrm{K}}_x\mathrm{F}{\mathrm{e}}₂\mathrm{A}{\mathrm{s}}₂$ using angle-resolved photoemission spectroscopy (ARPES), and we found an anomaly in the superconducting (SC) gap structure only for $x=0.25$ in both the hole and electron Fermi surfaces. We also propose that the mechanism of a newly observed double-resonance peak by inelastic neutron scattering is understandable because of the SC gap obtained by ARPES. Our discovery reveals the important relationship between the ${C}₄$-magnetic phase and superconductivity, and it provides an opportunity to survey the underlying relationship between the SC gap and the resonance mode in the hole-doped iron-based superconductor.