Critical Phenomena of Surface Phase Transitions: Theoretical Studies of the Structure Factor
Great progress has been made in the theory of the critical properties of phase transitions of lat... more Great progress has been made in the theory of the critical properties of phase transitions of lattice gases in two dimensions, but nature has provided very few realizations. Systems of chemisorbed atoms may well offer examples of the small number of universality classes, but one must worry about whether the limited size of defect-free regions precludes close enough approach to the critical regime to obtain scaling over a reasonable range and so whether the measured, effective exponents resemble the expected pure exponents. These concerns were heightened by the disagreement between expected and measured exponents for 0/Ni(111) [1]. To check what might be measured under ideal circumstances, we performed extensive Monte Carlo calculations [2–5] on lattices of about 4000 sites, comparable to the size of defect-free regions on metal surfaces. We assumed periodic boundary conditions and used lattice boundaries with the full symmetry of the substrate, i.e. hexagonal for triangular lattices...
Measurement of the Specific Heat Critical Exponent Using LEED
The Structure of Surfaces, 1985
Near a second-order phase boundary, integrated intensities of “extra” LEED beams exhibit a |T -Tc... more Near a second-order phase boundary, integrated intensities of “extra” LEED beams exhibit a |T -Tc|1-α singularity, where α is the specific heat critical exponent. We discuss the origin of this effect, apply it to real and Monte-Carlo-generated data, and comment on generalizations.
We study the effect of hindered aggregation and/or nucleation on the island formation process in ... more We study the effect of hindered aggregation and/or nucleation on the island formation process in a two-step growth protocol. In the proposed model, the attachment of monomers to islands and/or other monomers is hindered by additional energy barriers which decrease the hopping rate of the monomers to the occupied sites of the lattice. For zero and weak barriers, the attachment is limited by diffusion while for strong barriers it is limited by reaction. We describe the time evolution of the system in terms of the monomer and island densities, N1 and N . We also calculate the gap length, the capture zone and the island distributions. For all the sets of barriers considered, the results given by the proposed analytical model are compared with those from kinetic Monte Carlo simulations. We found that the behavior of the system depends on the ratio of the nucleation barrier to the aggregation barrier. The two-step growth protocol allows more control and understanding on the island formation mechanism because it intrinsically separates the nucleation and aggregation processes in different times regimes.
Abstract: Extended fine structure above the vanadium 2s appearance potential edge
Journal of Vacuum Science and Technology, 1978
Summary Abstract: O/Ni(111): Adlayer phases and binding sites
Journal of Vacuum Science and Technology, 1980
Fluctuations of Step Edges: Revelations About Atomic Processes Underlying Surface Mass Transport
MRS Proceedings, 1998
Experimental advances in recent years make possible quantitative observations of step-edge fluctu... more Experimental advances in recent years make possible quantitative observations of step-edge fluctuations. By applying a capillary-wave analysis to these fluctuations, one can extract characteristic times, from which one learns about the mass-transport mechanisms that underlie the motion as well as the associated kinetic coefficients [1-3]. The latter do not require a priori insight about the microscopic energy barriers and can be applied to situations away from equilibrium. We have studied a large number of limiting cases and, by means of a unified formalism, the crossover between many of these cases[4]. Monte Carlo simulations have been used to corroborate these ideas. We have considered both isolated steps and vicinal surfaces; illustrations will be drawn from noble-metal systems, though semiconductors have also been studied. Attachment asymmetries associated with Ehrlich-Schwoebel barriers play a role in this behavior. We have adapted the formalism for nearly straight steps to nea...
Using the embedded atom method (EAM) we compute the energies of principal steps, kinks, and singl... more Using the embedded atom method (EAM) we compute the energies of principal steps, kinks, and single-layer clusters of adatoms (islands) and of vacancies for Ag{lOO) and Ag(ll1). The energies are semiquantitatively consistent with experiments. Comparisons are frequently made with estimates based on nearest-neighbor bond counting. On Ag{lll) and Pt(ll1) the ratio of the energies of the two close-packed steps is closer to unity than measured in experiments on Pt. The energies of clusters arc essentially proportional to their perimeter, providing an easy way to estimate the binding energy of clusters to step edges. Adatom-vacancy symmetry is a good approximation except for single-site defects. Our calculations of barriers for single-atom diffusion near steps, compared to across terraces, are consistent with the fractal-like fingered growth of islands experimentally observed on (111) but not seen on (100). Computed spring constants of surface atoms suggest small changes in perpendicular vibration frequencies near step edges.
In recent observations of Brownian motion of islands of adsorbed atoms and of vacancies with mean... more 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 have investigated the step stiffness on Cu(001) surfaces as a function of step orientation by ... more We have investigated the step stiffness on Cu(001) surfaces as a function of step orientation by two independent methods at several temperatures near 300 K. Both sets of data agree well and show a substantial dependence of the stiffness on the angle of orientation. With the exception of steps oriented along 110 , the experimental stiffness is significantly larger than the stiffness calculated within the solid-on-solid (SOS) model and the Ising-model, even if next nearest-neighbor interactions are taken into account. Our results have considerable consequences for the understanding and for the theoretical modeling of equilibrium and growth phenomena, such as step meandering instabilities.
With the great success of EXAFS and related X-ray techniques, it is natural to ask why one should... more With the great success of EXAFS and related X-ray techniques, it is natural to ask why one should bother with a technique that replaces X-rays by incident electrons, sacrificing an intrinsic dipole selection rule and convenient polarization. There are several good reasons for using appearance potential spectroscopy (APS): First, it opens the possibility of absorption fine structure measurement to any modern surface science laboratory. The technique can use single crystal samples, is intrinsically surface sensitive [in most modes of operation], and is ipso facto ultra-high vacuum compatible. The only equipment needed is a low energy electron diffraction system or, better, a cylindrical mirror analyzer (CMA). Second, in a recently developed mode (AMEFS), the Auger decay of a particular element can be selectively monitored. This procedure opens the possibility of studying systems previously inaccessible because fine structure above different core levels overlapped. Third, the excitation matrix element for electron excitation is such that the decay of the fine structure is very slow as a function of energy above threshold. Thus, one obtains excellent data ranges, extending up to 11 or 12 A−1. Accordingly, the first 100 eV can be excluded from the analysis, thereby avoiding the regime where calculated phase shifts are least reliable.
Step Fluctuations: From Equilibrium Analysis to Step Unbunching and Cluster Diffusion in a Unified Picture
The electrochemical cell provides a potentially powerful means of altering morphology and islandi... more The electrochemical cell provides a potentially powerful means of altering morphology and islanding phenomena on metallic surfaces. Diffusion and attachment processes on terraces and near step and island edges are known to profoundly affect island sizes, shapes and coarsening kinetics. Using the surface-embedded-atom-model (SEAM) for describing metallic surfaces in the electrolytic environment, we calculate the dependence of the activation energies for the aforementioned diffusion processes on the deposited surface charge for the Ag(111) and Ag(100) surfaces in an electrolytic environment. While all these processes show some degree of dependence on the potential, the step-edge barrier and the edge diffusion processes are the most sensitive. Step-edge barriers increase (to over 1 eV) with large positive potential (0.85 V), while edge diffusion barriers monotonically decrease with positive surface charge on Ag(100) and Ag(111). We assess the effect these diffusion barriers have on isl...
Using reflection electron microscopy we examine the step fluctuations of Si͑111͒ at 1100 °C. Evap... more Using reflection electron microscopy we examine the step fluctuations of Si͑111͒ at 1100 °C. Evaporation is compensated by a replenishing flux. The step fluctuation behavior is qualitatively similar to that at 900 °C ͑where sublimation is negligible͒, with unexplained quantitative differences. We focus on the three parameters of the step continuum model of vicinals. The step stiffness scales with an increase in T from 900 °C as predicted by an appropriate lattice model. The kinetic coefficient is larger than scaling of the parameters from 900 °C would predict. The step-step correlations are assessed in traditional and novel ways; step repulsions are at least 6 times as strong as predicted from lower temperatures, suggesting nonequilibrium effects probably due to electromigration.
We present a method of analyzing the results of density functional modeling of molecular adsorpti... more We present a method of analyzing the results of density functional modeling of molecular adsorption in terms of an analogue of molecular orbitals. This approach permits intuitive chemical insight into the adsorption process. Applied to a set of anthracene derivates (anthracene, 9,10-anthraquinone, 9,10-dithioanthracene, and 9,10-diselenonanthracene), we follow the electronic states of the molecules that are involved in the bonding process and correlate them to both the molecular adsorption geometry and the species' diffusive behavior. We additionally provide computational code to easily repeat this analysis on any system.
Page 1. VOLUME 71, NUMBER 23 PHYSICAL REVIEW LETTERS Novel Critical Behavior in Inhomogeneous Sys... more Page 1. VOLUME 71, NUMBER 23 PHYSICAL REVIEW LETTERS Novel Critical Behavior in Inhomogeneous Systems 6 DECEMBER 1993 A. L. Stella, ' Michael R. Swift, Jacques G. Amar, TL Einstein, MW Cole, and Jayanth ...
Adsorption on metal surfaces: Final report
This report discusses the progress at the University of Maryland Department of Physics on the ads... more This report discusses the progress at the University of Maryland Department of Physics on the adsorption of atoms or molecules on the surfaces of metals. Also discussed are: Phase transformation studies; the use of transfer matrices to study the 2-d, 3-state chiral Potts model; electron-induced ionization of core electrons of atoms; the reflected electron energy loss fine structure above the M/sub 2,3/ core excitation edge of Cu; and other research in atomic and solid state physics. (LSP)
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