Here, 3He spin-echo dynamics measurements were made of low coverages of Cs on Cu(001). This revealed quasi-elastic broadening of the He beam, due to aperiodic transport on the surface, and extended measurements of the previously observed low-energy acoustic phonon mode at coverages of between 0.014 and 0.056ML at 130 or 80K. The low-energy phonons and quasi-elastic broadening occurred on similar time-scales, and the contributions were separated by converting the spin-echo measurements to the energy domain. Langevin molecular dynamics simulations reproduced the variation in the quasi-elastic peak-width, phonon position and amplitudes with momentum transfer, temperature and coverage. The main features of the experimental data required a potential corrugation of 20meV and a friction parameter of 1/40ps-1. The results indicated that the Cs dynamics were dominated by dipole-dipole repulsion, and produced strongly correlated motion. However, contrary to previous expectations, the transport proceeded via a jump-like behaviour within the Cs overlayer, and Cs moved much more freely, than did other alkali metals, on Cu. The unusual behaviour that was observed required 3 critical components. These were strong inter-adsorbate forces, a weak but finite substrate corrugation and a low adsorbate/substrate friction. Taken together, these key features were seen as a distinct signature in the intensity distribution across the energy/momentum exchange spectrum.

Vibration and Diffusion of Cs Atoms on Cu(001). A.P.Jardine, G.Alexandrowicz, H.Hedgeland, R.D.Diehl, W.Allison, J.Ellis: Journal of Physics - Condensed Matter, 2007, 19[30], 305010 (18pp)