Diffusion on a topological insulator surface: H2O on Bi2Te3(111)
Tamtögl1,2, N. Avidor1, I. Calvo-Almazán1, P. Townsend1, D. J. Ward1, M. Bianchi3, P. Hofmann3, J. Ellis1, W. Allison1 and W. E. Ernst2
1.The Cavendish Laboratory, Cambridge, United Kingdom
2.Institute of Experimental Physics, Graz University of Technology, Graz, Austria
3.Department of Physics and Astronomy, Aarhus University, Aarhus, Denmark
Water is ubiquitous in everyday life, yet the simplest mechanisms controlling its molecular motion at a surface have to be revealed by experiment[1-3]. We have studied the diffusion of H2O on the Bi2Te3(111) surface using helium-3 spin-echo spectroscopy, a reciprocal-space technique that places tracer and collective diffusion on the same footing. While it is known that water does not react with Bi2Te3, information about the diffusion of adsorbates on topological insulator surfaces is scarce. Notably, surface diffusion measurements are capable of providing new bench-mark data for energy landscapes on topological insulator surfaces with their peculiar electronic surface effects.
The diffusion of H2O on in-situ cleaved single crystals of Bi2Te3(111) was studied by applying a water overpressure in the temperature range of 135 – 160 K. The molecular dynamics extracted from spin-echo measurements shows thermally activated diffusion with a jump mechanism and an activation energy of 40 meV. The dependence upon the momentum transfer is characteristic for jumps on a hexagonal lattice in accordance with the preferred adsorption site from density functional theory calculations. Furthermore, the measurements indicate that interactions between the individual water molecules play a significant role in the diffusion mechanism of water.
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