Broken relationship between superconducting pairing interaction and electronic dispersion kinks in La_2-xSr_xCuO₄ measured by angle-resolved photoemission

Electronic band dispersions in copper oxide superconductors have kinks around 70 meV that are typically attributed to coupling of electrons to a bosonic mode. We performed angle-resolved photoemission spectroscopy experiments on overdoped cuprate high temperature superconductors to test the relationship between the superconducting transition temperature and electron-bosonic mode coupling. Remarkably, the kinks remain strong in the heavily overdoped region of the doping phase diagram of La_2-xSr_xCuO₄, even when the superconductivity completely disappears. This unexpected observation is incompatible with the conventional picture of superconductivity mediated by the sharp bosonic modes that are responsible for the kink unless extra doping strongly suppresses their d-wave projection but not the overall spectral weight. Our results favor pairing mediated by a very broad electronic spectrum or an unconventional mechanism without pairing glue.