Qualitative observations of vacancy-induced surface diffusion

Qualitative observations of vacancy-induced surface diffusion 

The first qualitative observation of vacancy-induced motion of embedded atoms was published in 1997 by Flores et al. [20], Using STM, an unusual, low mobility of embedded Mn atoms in Cu(0 0 1) was observed. Flores et al. argued that this could only be consistent with a vacancy-mediated diffusion mechanism. Upper and lower limits for the jump rate were established in the low-coverage limit and reasonable agreement was obtained between the experimentally observed diffusion coefficient and a theoretical estimate based on vacancy-mediated diffusion. That same year it was proposed that the diffusion of vacancies is the dominant mechanism in the decay of adatom islands on Cu(00 1) [36], which was also backed up by ab initio calculations [37]. After that, studies were performed on the vacancy-mediated diffusion of embedded In atoms [21-23] and Pd atoms [24] in the same surface. The deployment of a high-speed variable temperature STM in the case of embedded In and an atom-tracker STM in the case of Pd, allowed for a detailed quantitative investigation of the vacancy-mediated diffusion process by examining in detail both the jump frequency as well as the displacement statistics. Experimental details of both setups have been published elsewhere [34,35]. A review of the quantitative results from these studies is presented in the next subsections. 

The spreading of the indium atoms through the first layer implies that they are able to diffuse, whilst remaining embedded within the surface. The diffusion behavior of the embedded indium atoms was studied in series of images of selected areas on the copper surface, forming STM movies of the motion. 

The diffusion behavior of the indium atoms in these movies differs from the usual hopping mechanism in several respects, as is illustrated by the STM images in Fig. 2. 

For exchange with a copper adatom the diffusion mechanism is illustrated in Fig. 3. In the measurements, embedded indium atoms are observed to make jumps of several atomic spacings. For the adatom mechanism, this would imply that the indium adatoms reinsert themselves into the terrace after making 

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