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Surface diffusion and dynamics on Dirac materials. (Please notice the unusual time!)

Wednesday, October 11, 2017 - 16:00
Place: 
Donostia International Physics Center
Who: 
Dr. Anton Tamtögl, Institute of Experimental Physics, Graz University of Technology, Graz, Austria
Source Name: 
DIPC

We have
studied the diffusion of water on the surfaces of Dirac materials (graphene and
the topological insulator Bi2Te3) using helium-3
spin-echo spectroscopy[1]. While water is ubiquitous in everyday life, the
simplest mechanisms controlling its molecular motion at a surface have yet to
be revealed by experiment[2-4]. Our experimental results provide a detailed
insight into the behaviour of H2O molecules on the surfaces of Dirac
materials.

Diffusion occurs
by an activated hopping motion where in particular in the case of graphene a
strong signature for cooperative behaviour due to repulsive forces between the
molecules appears. The diffusion of water on Bi2Te3 is
governed by a large atomic-scale friction between the adsorbate and the
substrate.

I will also
present some details on the surface dynamics of pristine topological insulator
surfaces. The atom-surface scattering probabilities can be used to extract the
electron-phonon coupling of topological insulator surfaces[5] and inelastic
helium atom scattering measurements are able to resolve the acoustic surface
phonon modes of Bi2Te3. Here the low energy region of the
lattice vibrations is mainly dominated by the Rayleigh mode which has been
claimed to be absent in previous experimental studies.

[1] A. Jardine et al., Prog. Surf.
Sci. 84, 323 (2009).

[2] A. Hodgson et al., Surf. Sci.
Rep. 64, 381 (2009).

[3] S.-C. Heidorn et al. ACS Nano 9,
3572 (2015).

[4] J. Carrasco et al., Nat. Mater.
11, 667 (2012).

[5] A. Tamtögl et
al.
, Phys. Rev. B. 95. 195401 (2017).


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