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We have identified a new defect at the intersection between grain boundaries and surfaces in Au using atomic resolution transmission electron microscopy. At the junction line of 90 degrees <110> tilt grain boundaries of (110)-(001) orientation with the free surface, a small segment of the grain boundary, about 1 nm in length, dissociates into a triangular region with a chevronlike stacking disorder and a distorted hcp structure. The structure and stability of these defects are confirmed by atomistic simulations, and we point out the relationship with the one-dimensional incommensurate structure of the grain boundary.

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We introduce a class of Monte Carlo algorithms that solve a dynamic problem defined by the transition rates and the initial state of a discrete system. This class contains the method of Bortz, Kalos, and Lebowitz ͑BKL͒ ͓J. Comp. Phys. 17, 10 ͑1975͔͒ as a limit. We show that introducing a constant time step in a Metropolis algorithm leads to an approximation of the solution in which the system relaxation times are underestimated. This can be corrected if the time step is an adequate stochastic variable. Thus, we are able to define kinetic Metropolis algorithms and generalize them in a case of nonconstant numbers of attempt configurations. The algorithm class allows us to introduce a useful method in which the calculation of transition rates are exploited for the next step in an adaptive way. This method corresponds to a kinetic Metropolis algorithm when the rejection probability is reasonable and becomes similar to the BKL method otherwise. We describe and compare four different algorithms applied to a physical example about the diffusion in lattice gases.

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The structure of thin Ni f i l m on Au(OO1) and Au(OO1)iNi multilayers is studied as a function of the thickness of Ni using a semiempirical potential based on the tight-binding second-moment approximation. It is shown that the stable structure is pseudomorphic for a thickness less than five monolayers and [ZiiOl hcp or 4H for a greater thidoless with closepacked planes perpendicular to the interfaces. Moreover. we show that a thin Ni film gmwn on Au(0Ol) substrate is covered by at least one monolayer of Au. We compare ow results with high-resolution electronic microscopy (HREM) and x-ray diffraction.

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