Quasiparticle dynamics on the topological surface state of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, ${\mathrm{Bi}}_2{\mathrm{Te}}_3$, and superconducting ${\mathrm{Cu}}_x{\mathrm{Bi}}_2{\mathrm{Se}}_3$ 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 $\sim 3$ and $\sim 15–20\mathrm{meV}$ associated with an exceptionally large coupling constant $\lambda$ of $\sim 3$, 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 ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, but it appears that the theoretically proposed “spin-plasmon” excitations realized in helical metals are also playing an important role. Intriguingly, the $\sim 3\mathrm{meV}$ mode coupling is found to be enhanced in the superconducting state of ${\mathrm{Cu}}_x{\mathrm{Bi}}_2{\mathrm{Se}}_3$.

• Received 29 December 2012

DOI:https://doi.org/10.1103/PhysRevLett.110.217601