Poster

Measuring the Sb(111)-phonon dispersion using HAS: Evidence for an extremely low-lying acoustic plasmon mode

P. Kraus¹, F. Apolloner¹, C. Gösweiner¹, G. Benedek^2,3, and W. E. Ernst¹

¹Institute of Experimental Physics, Graz University of Technology, Graz, Austria

²Donostia International Physics Centre (DIPC), San Sebastián/Donostia, Spain

³Dipartimento di Scienza die Materiali, Universitá di Milano-Bicocca, Milano, Italy

As one of the essential components in the recently discovered group of topological insulators [1], the semimetal antimony (Sb) is an attractive candidate for studying the properties of its surface. Quite recently the surface phonon dispersion curve of Sb(111) as well as the influences of electron-phonon interactions were calculated using density functional perturbation theory [2]. However, there has been a shortage of experimental data for comparison and verification of the models used. Hence, for a comprehensive understanding of topological insulators, measurements on antimony are long overdue.

As a surface sensitive technique, Helium Atom Scattering (HAS) allows investigating elastic as well as inelastic phenomena associated with structure and dynamics of the electronic corrugation slightly above the Sb(111) surface [3,4,5].

Recent inelastic HAS measurements on Sb(111) revealed not only the expected surface phonon dispersion, but also multiple unexpected branches, one of which can be assigned to a low-lying acoustic plasmon mode while the other, lying completely below the Rayleigh curve of the phonon dispersion with a maximum energy at the K-point below 3 meV, can not be assigned to any known feature up to now.

The electronic similarity of Sb(111) with a single graphene layer, both containing two distinguishable groups of three Dirac cones in a hexagonal arrangement suggests that this low lying mode is indeed an additional intervalley plasmon mode such as has been observed in graphene [6].

HAS experimental data including the complete surface phonon dispersion as well as the isolated low plasmon mode will be presented.

[1] H. Zhang et al., Nature Physics 5, 438-442 (2009)

[2] D. Campi et al., Phys. Rev. B. 86, 075446 (2012)

[3] M. Mayrhofer-R. et al., J. Phys. Condens. Matter 25, 395002 (2013)

[4] M. Mayrhofer-R. et al., Phys. Rev. B 88, 205425 (2013)

[5] P. Kraus et al., Phys. Rev. B 87, 245433 (2013)

[6] T. Tudorovski et al., Phys. Rev. B 82 073411 (2010)