  A recent highlight in the study of high-Tc superconductors is the observation
of band renormalization / self-energy effects on the quasiparticles. This is
seen in the form of kinks in the quasiparticle dispersions as measured by
photoemission and interpreted as signatures of collective bosonic modes
coupling to the electrons. Here we compare for the first time the self-energies
in an optimally doped and strongly overdoped, non-superconducting single-layer
Bi-cuprate (Bi2Sr2CuO6). Besides the appearance of a strong overall weakening,
we also find that weight of the self-energy in the overdoped system shifts to
higher energies. We present evidence that this is related to a change in the
coupling to c-axis phonons due to the rapid change of the c-axis screening in
this doping range.
