Poster

Acoustic surface plasmons and phonon-plasmon coupling in Cu(111) from inelastic He-atom scattering

G. Benedek1-3, E. V. Chulkov2, V. M. Silkin2, P. M. Echenique2, J. P. Toennies3, and G. Zhang3

1Dipartimento di Scienza dei Materiali, Universitá di Milano-Bicocca, Via R. Cozzi 53, 20125 Milano, Italy

2Donostia International Physics Center (DIPC), University of the Basque Country (UPV-EHU) 20018 Donostia/San Sebastian, Spain

3Max-Planck-Institut für Dynamik und Selbstorganisation, Bunsenstraße 10, 37073 Göttingen, Germany

Inelastic Helium atom scattering (HAS) spectroscopy from a conducting surface can excite surface and subsurface phonons only via the phonon-induced modulation of the surface charge density, thus providing a direct measurement mode-selected electron-phonon coupling strength (mode-lambda spectroscopy) [1,2]. The recent discovery of acoustic surface plasmons (ASP) at metal surfaces associated with shallow surface electronic states [3,4] raises the natural question whether HAS spectroscopy can directly detect ASPs. A new analysis of the inelastic HAS spectra from the (111) surface of copper reveals a series of sharp inelastic features whose dispersion can be ascribed to that expected for ASPs in the extreme acoustic limit. In the THz domain the ASPs dispersion curves cross the surface-projected bulk phonon density and the dispersion curves of optical surface phonons. The ASP-phonon coupling leads to avoided crossing with the optical surface phonon branches and to peculiar Fano shapes of the ASP peaks due to the overlap of the ASP dispersion curve with the surface-projected bulk phonon continuum.

[1] I. Yu. Sklyadneva, G. Benedek, E. V. Chulkov, P. M. Echenique, R. Heid, K.-P. Bohnen and J. P. Toennies, Phys. Rev. Lett. 107, 095502 (2011)

[2] G. Benedek, M. Bernasconi, K.-P. Bohnen, D. Campi, E. V. Chulkov, P. M. Echenique, R. Heid, I. Yu. Sklyadneva and J. P. Toennies, Phys. Chem. Chem. Phys. 16, 7159 (2014)

[3] V. M. Silkin, J. M. Pitarke, E. V. Chulkov, and P. M. Echenique, Phys. Rev. B 72 115435 (2005)

[4] B. Diaconescu, K. Pohl, L. Vattuone, L. Savio, Ph. Hofmann, V. M. Silkin, J. M. Pitarke, E. V. Chulkov, P. M. Echenique, D. Farías, and M. Rocca, Nature 57, 448 (2007)