Terahertz surface modes and electron-phonon coupling on Bi2Se3(111)

We present a combined experimental and theoretical study of the surface vibrational modes of the topological insulator Bi2Se3 with particular emphasis on the low-energy region below 10 meV that has been difficult to resolve experimentally. By applying inelastic helium atom scattering (HAS), the entire phonon dispersion was determined and compared with density functional perturbation theory (DFPT) calculations. The intensity of the phonon modes is dominated by a strong Rayleigh mode, in contrast to previous experimental works. Moreover, also at variance with recent reports, no Kohn anomaly is observed. These observations are in excellent agreement with DFPT calculations. Besides these results, the experimental data reveal-via bound-state resonance enhancement-two additional dispersion curves in the gap below the Rayleigh mode. They are possibly associated with an excitation of a surface electron density superstructure that we observe in HAS diffraction patterns. The electron-phonon coupling parameter lambda = 0.23, derived from our temperature-dependent Debye-Waller measurements, compares well with values determined by angular resolved photoemission or Landau level spectroscopy. Our work opens up a new perspective for terahertz (THz) measurements on two-dimensional (2D) materials as well as the investigation of subtle details (band bending, the presence of a 2D electron gas) with respect to the electron-phonon coupling.