Special photon energies for extracting the bosonic spectral function mediating superconductivity in ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_8$ via angle-resolved photoemission spectroscopy

We delineate the complex nature of the angle-resolved photoemission spectroscopy (ARPES) matrix element in ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_8$ and identify photon energies where the matrix element is insensitive to wave vector and/or frequency. These special photon energies provide a unique route for extracting the spectral function of the bosonic glue mediating superconductivity and for obtaining self-energies more generally via ARPES experiments.