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

Interaction of Phonons and Dirac Fermions on the Surface of Bi2Se3: A Strong Kohn Anomaly