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Interaction of phonons and Dirac fermions on the surface of Bi2Se3: A strong Kohn anomaly

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Strong TIs have exotic metallic surface states protected by time-reversal invariance (TRI). The linear dispersion of these surface states, Dirac cones, depicts massless Dirac fermion quasi-particles with definite helicity, whereby the spin is locked normal to the wavevector of the electronic state. Helium atom/surface scattering measurements of phonon dispersion curves on the (001) surface of the strong three-dimensional topological insulator Bi2Se3 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.

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