  Quasiparticle dynamics on the topological surface state of Bi2Se3, Bi2Te3,
and superconducting CuxBi2Se3 are studied by 7 eV laser-based angle resolved
photoemission spectroscopy. We find strong mode-couplings in the Dirac-cone
surface states at energies of ~3 and ~15-20 meV, which leads to an
exceptionally large coupling constant of ~3, which is one of the strongest ever
reported for any material. This result is compatible with the recent
observation of a strong Kohn anomaly in the surface phonon dispersion of
Bi2Se3, but it appears that the theoretically proposed "spin-plasmon"
excitations realized in helical metals are also playing an important role.
Intriguingly, the ~3 meV mode coupling is found to be enhanced in the
superconducting state of CuxBi2Se3.
