Interaction of Phonons and Dirac Fermions on the Surface ofBi2Se3: A Strong Kohn Anomaly

Abstract

We report the first measurements of phonon dispersion curves on the (001) surface of the strong three-dimensional topological insulator Bi 2 Se 3 . 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 2k F 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.

Electron-phonon interaction on the surface of a three-dimensional topological insulator

“…In this context, the electron-phonon (e-ph) interaction at the surface of a three-dimensional (3D) TI is particularly relevant, as it could make a significant contribution to the transport properties of the surface states. [3][4][5][6] Experimental evidence is ambiguous. Angleresolved photoemission spectroscopy (ARPES) measurements reveal small effects on surface electrons, 6 suggesting that there is a negligible interaction between electrons and surface phonons at the boundary of a 3D TI.…”

“…Angleresolved photoemission spectroscopy (ARPES) measurements reveal small effects on surface electrons, 6 suggesting that there is a negligible interaction between electrons and surface phonons at the boundary of a 3D TI. On the other hand, a strong coupling of phonons with surface electrons is extracted from measurements of surface phonon spectra in Bi 2 Se 3 , leading to a strong Kohn anomaly for transverse optical surface phonons, while there is no evidence of a Rayleigh acoustical branch 4 which would usually be expected to exist in systems with a free surface. 5 A possible explanation given for the latter feature is a different coupling between planes in the quintuple layer of Bi 2 Se 3 .…”

Surface phonons in the topological insulators Bi2Se3 and Bi2Te3

“…Because of its crucial relevance to their surface conductivity properties, the study of electron-phonon coupling [13] and, moreover, the search for vibrations localized at the surface have been the subject of many studies in recent years [14,15,16,17]. In particular, inelastic helium scattering [14,15], surface enhanced Raman scattering [16] and time-resolved photoemission measurements [17] in Bi2Se3 and Bi2Te3 show features that were attributed to surface modes as well as strong electron-phonon coupling at the surface.…”

“…Because of its crucial relevance to their surface conductivity properties, the study of electron-phonon coupling [13] and, moreover, the search for vibrations localized at the surface have been the subject of many studies in recent years [14,15,16,17]. In particular, inelastic helium scattering [14,15], surface enhanced Raman scattering [16] and time-resolved photoemission measurements [17] in Bi2Se3 and Bi2Te3 show features that were attributed to surface modes as well as strong electron-phonon coupling at the surface.Also, weak features observed in Raman spectra were attributed to surface effects unrelated to the topological surface states [18].Here, we present experimental results on the temperature-and excitation-wavelength (λL−) dependence of Raman scattering, as well as first-principles phonon calculations for bulk and fewquintuple-layer Bi2Se3 and Bi2Te3. Other than the expected, and previously reported Raman-active bulk modes [19,20], we find weak peaks at low temperatures in both compounds, which we ascribe 2 | P a g e to surface vibrational modes.…”

Helium-surface interaction potential of Sb(111) from scattering experiments and close-coupling calculations

“…19,20 Furthermore, e-ph scattering processes at the Fermi level are likely to cause Kohn anomalies in the surface phonon dispersion, 19,20,25 an effect which has been observed with HAS for the topological insulator surfaces Bi 2 Se 3 and Bi 2 Te 3 . [26][27][28] In the present work the elastic diffraction peaks of He scattered from Sb(111) are analyzed using the exact closecoupling (CC) method. 15 This method goes beyond the simple hard-wall approximation and has been mainly applied to ionic and metallic surfaces.…”