In recent observations of Brownian motion of islands of adsorbed atoms and of vacancies with mean radius R, the cluster diffusion constant varies as R 21 and R 22. From an analytical Langevin description of the cluster's steplike boundary, we find three cases, R 21 , R 22 , and R 23 , corresponding to the three microscopic surface mass-transport mechanisms of straight steps. We thereby provide a unified treatment of the dynamics of steps and of clusters. For corroboration, we perform Monte Carlo simulations of simple lattice gases and derive atomistic diffusion constants.

In recent observations of Brownian motion of islands of adsorbed atoms and of vacancies with mean radius R, the cluster diffusion constant D c varies as R Ϫ1 and R Ϫ2 . From an analytical continuum description of the cluster's steplike boundary, we find a single Langevin equation for the motion of the cluster boundary, rather than three special cases. From this we determine D c and the correlation function G sh for fluctuations of the shape around an assumed equilibrium circular shape. In three limiting cases we find the scaling relations D c ϳR Ϫ␣ and, at early elapsed time t, G sh ϳt 1/(1ϩ␣) , where ␣ϭ1, 2, and 3, corresponding to the three generic surface mass-transport mechanisms of straight steps. We thereby provide a unified treatment of the dynamics of steps and of clusters. To check how well the continuum results describe clusters of the size in experiments, we perform Monte Carlo simulations of simple lattice gas models. Further, we estimate atomic diffusion parameters from the available experimental data on diffusion of large clusters.

We study theoretically the equilibrium fluctuations of steps on vicinal surfaces. From an analytical continuum description of the step edge, we find a single Langevin equation governing the motion of an isolated step around its equilibrium position that includes attachment/detachment of atoms, diffusion over the terrace, diffusion along the edge, and evaporation. We then extend this approach to treat an array of steps, i.e., a vicinal surface. We also present, in an appendix, an alternative formalism in which detachment to terrace and to step-edge diffusion can take place independently. In established as well as some new limits, and for numerous special cases, we study the wave-vector dependence-both exponent and prefactor-of the relaxation time of fluctuations. From this we recover scaling relations for early-time dependence of the mean-square fluctuations. We discuss how to extract the ͓mesoscopic͔ transport coefficients associated with different atomistic mechanisms of surface mass transport and how to distinguish between mechanisms having the same power-law dependence on wavelength in the capillary-wave analysis. To examine the crossovers between limiting regimes, we compute and explore an effective exponent for this power law and show that the crossover occurs over a narrow region of phase space. Furthermore, we find that single-sided approximations are valid only in the limit of extreme Schwoebel barriers. ͓S0163-1829͑98͒09008-0͔

The high-pressure structural and vibrational properties of Bi2S3 have been probed up to 65 GPa with a combination of experimental and theoretical methods. The ambient-pressure Pnma structure is found to persist up to 50 GPa; further compression leads to structural disorder. Closer inspection of our structural and Raman spectroscopic results reveals notable compressibility changes in specific structural parameters of the Pnma phase beyond 4-6 GPa. By taking the available literature into account, we speculate that a second-order isostructural transition is realized near that pressure, originating probably from a topological modification of the Bi2S3 electronic structure near that pressure. Finally, the Bi(3+) lone-electron pair (LEP) stereochemical activity decreases against pressure increase; an utter vanishing, however, is not expected until 1 Mbar. This persistence of the Bi(3+) LEP activity in Bi2S3 can explain the absence of any structural transitions toward higher crystalline symmetries in the investigated pressure range.

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