  We report the first measurements of phonon dispersion curves on the (001)
surface of the strong three-dimensional topological insulator Bi2Se3. The
surface phonon measurements were carried out with the aid of coherent helium
beam surface scattering techniques. The results reveal a prominent signature of
the exotic metallic Dirac fermion quasi-particles, including a strong Kohn
anomaly. The signature is manifest in a low energy isotropic convex dispersive
surface phonon branch with a frequency maximum of 1.8 THz, and having a
V-shaped minimum at approximately 2kF that defines the Kohn anomaly.
Theoretical analysis attributes this dispersive profile to the renormalization
of the surface phonon excitations by the surface Dirac fermions. The
contribution of the Dirac fermions to this renormalization is derived in terms
of a Coulomb-type perturbation model.
