The low-temperature surface diffusion of isolated Cr adatoms on Au(111) was determined using non-perturbing x rays. Changes in the x-ray magnetic circular dichroism spectral line shape together with Monte Carlo calculations demonstrated that adatom nucleation proceeds via quantum tunnelling diffusion rather than over-barrier hopping for temperatures below 40K. The jump rates were shown to be as much as 35 orders of magnitude higher than that expected for thermal over-barrier hopping at 10K.

Surface Diffusion of Cr Adatoms on Au(111) by Quantum Tunneling. P.Ohresser, H.Bulou, S.S.Dhesi, C.Boeglin, B.Lazarovits, E.Gaudry, I.Chado, J.Faerber, F.Scheurer: Physical Review Letters, 2005, 95[19], 195901