THz Surface Modes and Electron-Phonon Coupling in Bi$_2$Se$_3$(111)

Abstract

We present a combined experimental and theoretical study of the surface vibrational modes of the topological insulator (TI) 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 paramenter λ = 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 THz measurements on 2D materials as well as the investigation of subtle details (band bending, the presence of quantum well states) with respect to the electron-phonon coupling.

Experimental determination of surface thermal expansion and electron–phonon coupling constant of 1T-PtTe2

“…Recent theoretical studies showed how the thermal attenuation of the He specular peak from metal surfaces, described by the Debye-Waller exponent, can be directly related to the e-ph coupling constant λ [18][19][20]. This approach has also been recently adapted to derive the e-ph coupling constant for the case of degenerated semiconductors and chalogenides [21][22][23]. This parameter, that allows to quanti fy the strength of the interaction between electrons and phonons at surfaces, is crucial to understand the scattering process of the surface-state electrons that can jeopardize the performance of 2D materials in possible applications [18,20].…”