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Surface vibrational modes of the topological insulator Bi2Se3 observed by Raman spectroscopy

H.-H. Kung, M. Salehi, I. Boulares, A. F. Kemper, N. Koirala, M. Brahlek, P. Lošťák, C. Uher, R. Merlin, X. Wang, S.-W. Cheong, S. Oh, and G. Blumberg
Phys. Rev. B 95, 245406 – Published 9 June 2017
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  • INTRODUCTION
  • EXPERIMENTAL SETUP
  • RESULTS
  • DISCUSSION
  • CONCLUSION
  • ACKNOWLEDGMENTS
  • APPENDICES
    • References

    Abstract

    We present a polarization resolved Raman scattering study of surface vibration modes in the topological insulator Bi2Se3 single crystal and thick films. Besides the four Raman active bulk phonons, we observed four additional modes with much weaker intensity and slightly lower energy than the bulk counterparts. Using symmetry analysis, we assigned these additional modes to out-of-plane surface phonons. Comparing with first-principle calculations, we conclude that the appearance of these modes is due to c-axis lattice distortion and van der Waals gap expansion near the crystal surface. Two of the surface modes at 60 and 173 cm−1 are associated with Raman active A1g bulk phonon modes, the other two at 136 and 158 cm−1 are associated with infrared active bulk phonons with A2u symmetry. The latter become Raman allowed due to reduction of crystalline symmetry from D3d in the bulk to C3v on the crystal surface. In particular, the 158 cm−1 surface phonon mode shows a Fano line shape under resonant excitation, suggesting interference in the presence of electron-phonon coupling of the surface excitations.

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    • Received 17 November 2016

    DOI:https://doi.org/10.1103/PhysRevB.95.245406

    ©2017 American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    Electron-phonon couplingOptical phononsSurface & interfacial phenomenaTopological insulators
    1. Physical Systems
    Multilayer thin filmsSingle crystal materialsSurfacesTransition metal dichalcogenides
    1. Techniques
    Raman spectroscopy
    Condensed Matter, Materials & Applied Physics

    Authors & Affiliations

    H.-H. Kung1,*, M. Salehi1,2, I. Boulares3, A. F. Kemper4, N. Koirala1, M. Brahlek1, P. Lošťák5, C. Uher3, R. Merlin3, X. Wang1,6, S.-W. Cheong1,6, S. Oh1, and G. Blumberg1,7,†

    • 1Department of Physics & Astronomy, Rutgers University, Piscataway, New Jersey 08854, USA
    • 2Department of Materials Science and Engineering, Rutgers University, Piscataway, New Jersey 08854, USA
    • 3Department of Physics, University of Michigan, Ann Arbor, Michigan 48109-1040, USA
    • 4Department of Physics, North Carolina State University, Raleigh, North Carolina 27695, USA
    • 5Faculty of Chemical Technology, University of Pardubice, Studentska 573, 53210 Pardubice, Czech Republic
    • 6Rutgers Center for Emergent Materials, Rutgers University, Piscataway, New Jersey 08854, USA
    • 7National Institute of Chemical Physics and Biophysics, 12618 Tallinn, Estonia

    • *skung@physics.rutgers.edu
    • †girsh@physics.rutgers.edu

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    Issue

    Vol. 95, Iss. 24 — 15 June 2017

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    Images

    • Figure 1
      Figure 1

      The Raman response function χ″(ω) measured in the (a) RR and (b) RL scattering geometry at 13 K with 532 nm excitation from various Bi2Se3 samples as described in Table 1, plot in semi-log scale. The dashed lines label the observed phonon modes as tabulated in Table 3. (a) The mode at 110 cm−1 indicated by an arrow is due to the phonon signal from α−In2Se3 layers [61]. The asterisks mark the phonon modes with A1g and A1 symmetries, and appear in RL geometry due to indium atom diffusion. The instrumental resolution of 2.8 cm−1 is shown.

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    • Figure 2
      Figure 2

      The signal intensity in the XX scattering geometry, measured at 10 K from a bulk Bi2Se3 single crystal, plot in semi-log scale. The blue, green, and pink lines corresponds to laser excitation energy of 476, 514, and 780 nm, respectively. Inset: Enlarged plot around the A1(3) mode. The black lines are fit to Fano line shape [Eq. (2)].

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    • Figure 3
      Figure 3

      (a) The Raman spectra taken in all four scattering geometries at 13 K with 532 nm excitation from a Bi2Se3 thick film, plotted on a semi-log scale. (b) The Raman response of different symmetry channels, obtained from data in (a). The bulk phonons are marked by dashed lines, whereas the surface modes are indicated by arrows and shaded in red.

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    • Figure 4
      Figure 4

      The Raman spectra taken in the (a) RR and (b) RL scattering geometry at 13 K with 532 nm excitation from a bulk Bi2Se3 single crystal are plotted on a semi-log scale. The red and blue curves correspond to instrumental resolution of 2.8 and 0.9 cm−1 [as shown in (b)], respectively. The bulk phonons are marked by gray dashed lines.

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    • Figure 5
      Figure 5

      Comparison of raw data and polarization leakage removed spectra, taken in (a) YX, (b) RL, (c) XX, and (d) RR polarization geometry from the ab surface of sample 2 at 300 K, with 532 nm excitation.

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