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

Adrian Ruckhofer,Martin Bremholm,Anton Tamtögl,Davide Campi,Giorgio Benedek,Marco Bernasconi,Wolfgang E Ernst,Philip Hofmann

doi:10.1103/physrevresearch.2.023186

Abstract

We present a combined experimental and theoretical study of the surface vibrational modes of the topological insulator (TI) Bi$_2$Se$_3$ 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 $\lambda$ = 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 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.

Highlights

Bi2Se3 (Fig. 1) is categorized as a three-dimensional topological insulator (TI), a new state of quantum matter with a bulk gap and spin-orbit split surface states forming a Dirac cone across the gap [1,2]
We recently observed with helium atom scattering (HAS) a multivalley charge density wave (CDW) in Sb(111) originating from the M-point electron pockets giving rise to additional peaks in the diffraction pattern [52]
We have determined the surface phonon dispersion curves of Bi2Se3 along both high-symmetry directions, where the largest inelastic scattering intensity is provided by the Rayleigh wave

Summary

INTRODUCTION

Bi2Se3 (Fig. 1) is categorized as a three-dimensional topological insulator (TI), a new state of quantum matter with a bulk gap and spin-orbit split surface states forming a Dirac cone across the gap [1,2]. Experimental information about the surface phonon dispersion curves of Bi2Se3(111) was limited to previous helium atom scattering (HAS) studies by Zhu et al [7,17], in the low-energy part of the phonon spectrum. These studies suggested the presence of a deep Kohn anomaly (KA) in the 7.5 meV optical phonon branch (S2) coupled to the electronic (spin-forbidden) transition across the Dirac cone [17]. The effect of SOC was found to be weak for the low-energy surface vibrational modes of typical TIs such as Bi2Te3 and Sb2Te3 [28,50], while on the other hand, it was shown that vdW corrections become important for an exact description of the low-energy optical modes of Bi2Te3 [28]

RESULTS AND DISCUSSION

Time-of-flight measurements and phonon dispersion curves

Low-energy branches

Electron-phonon coupling

CONCLUSIONS