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VOLUME 33 | ISSUE 6 | PAGE 326
Diffusion in two-dimensional crystals
In experiments on the diffusion of a submonolayer barium film on the Mo(l 10) face, Vedula et al.l'2 found that the diffusion coefficient D, the activation energy ea, and the coefficient (D0) of the exponential function are all nonmonotonic functions of the degree of surface coverage θ (the ratio of the actual concentration of adatoms to the concentration in a closely packed monolayer). Earlier studies of structures in this system3 had demonstrated the existence of several commensurable phases at small values of Θ and of an incommensurable phase at θ >0.6. Comparison of structural data with data on diffusion reveals a correlation. In particular, ea and Do have minima at coverages corresponding to the phases C(2 X 6) (0 = 0.8) and C(2 X 2) (0 = 0.6). The activation energy near these minima has clearly defined plateaus in an interval Δ0 ~ 0.1, while D0 increases by a factor of 103 10s in an interval1* Δ0~Ο.1.




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