Physical Sciences and Mathematics Commons^™

Dislocations And Vacancies In Two-Dimensional Mixed Crystals Of Spheres And Dimers, Sharon J. Gerbode, Desmond C. Ong, Chekesha M. Liddell, Itai Cohen

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In colloidal crystals of spheres, dislocation motion is unrestricted. On the other hand, recent studies of relaxation in crystals of colloidal dimer particles have demonstrated that the dislocation dynamics in such crystals are reminiscent of glassy systems. The observed glassy dynamics arise as a result of dislocation cages formed by certain dimer orientations. In the current study, we use experiments and simulations to investigate the transition that arises when a pure sphere crystal is doped with an increasing concentration of dimers. Specifically, we focus on both dislocation caging and vacancy motion. Interestingly, we find that any nonzero fraction of dimers …

Glassy Dislocation Dynamics In 2d Colloidal Dimer Crystals, Sharon J. Gerbode, Ugmang Agarwal, Desmond C. Ong, Chekesha M. Liddell, Fernando Escobedo, Itai Cohen

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Although glassy relaxation is typically associated with disorder, here we report on a new type of glassy dynamics relating to dislocations within 2D crystals of colloidal dimers. Previous studies have demonstrated that dislocation motion in dimer crystals is restricted by certain particle orientations. Here, we drag an optically trapped particle through such dimer crystals, creating dislocations. We find a two-stage relaxation response where initially dislocations glide until encountering particles that cage their motion. Subsequent relaxation occurs logarithmically slowly through a second process where dislocations hop between caged configurations. Finally, in simulations of sheared dimer crystals, the dislocation mean squared displacement …

Restricted Dislocation Motion In Crystals Of Colloidal Dimer Particles, Sharon J. Gerbode, Stephanie H. Lee, Chekesha M. Liddell, Itai Cohen

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Received 2 April 2008; published 1 August 2008; corrected 1 October 2008

At high area fractions, monolayers of colloidal dimer particles form a degenerate crystal (DC) structure in which the particle lobes occupy triangular lattice sites while the particles are oriented randomly along any of the three lattice directions. We report that dislocation glide in DCs is blocked by certain particle orientations. The mean number of lattice constants between such obstacles is Z̅ _exp=4.6±0.2 in experimentally observed DC grains and Z̅ _sim=6.18±0.01 in simulated monocrystalline DCs. Dislocation propagation beyond these obstacles is observed to proceed through dislocation reactions. …

The Structure Of Alkali Halide Dimers: A Critical Test Of Ionic Models And New Ab Initio Results, T. Törring, S. Biermann, J. Hoeft, Richard J. Mawhorter, Robert J. Cave, C. Szemenyei

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In semiempirical ionic models a number of adjustable parameters have to be fitted to experimental data of either monomer molecules or crystals. This leads to strong correlations between these constants and prevents a unique test and a clear physical interpretation of the fit parameters. Moreover, it is not clear whether these constants remain unchanged when the model is applied to dimers or larger clusters. It is shown that these correlations can be substantially reduced when reliable information about dimers is available from experiments or ab initio calculations. Starting with Dunham coefficients of the monomer potential determined from microwave measurements, we …

A Theoretical Investigation Of The Geometries, Vibrational Frequencies, And Binding Energies Of Several Mixed Alkali Halide Dimers, Robert J. Cave, Ian Ono '94

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Results are presented from ab initio calculations on several mixed alkali halide dimers made up of Li, Na, F, and Cl. All of the dimers are investigated at the restricted Hartree–Fock level to provide ab initio estimates of geometries, vibrational frequencies, and binding energies. The dimer LiNaF2 is then treated using a variety of basis sets at the Hartree–Fock level, as well as at correlated levels (second‐order Møller–Plesset and coupled‐cluster singles and doubles) to examine the sensitivity of the calculations to use of higher levels of theory. The results are then compared to the experimental data available for the LiNaF2 …

A Theoretical Investigation Of The Geometries, Vibrational Frequencies, And Binding Energies Of Several Alkali Halide Dimers, Robert P. Dickey '93, David Maurice '91, Robert J. Cave, Richard J. Mawhorter

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Results are presented from ab initio calculations on the symmetrical alkali halide dimers made up of Li, Na, K, F, and Cl. We examine the sensitivity of representative monomer and dimer geometries to the variation of the basis set with and without polarization and diffuse functions. The geometries are then compared with available experimental results. We have also calculated vibrational frequencies at the restricted Hartree–Fock level and examined the changes in geometry brought about by correlation using second‐order Møller–Plesset perturbation theory. It is found that Hartree–Fock theory in a modest basis set with diffuse and polarization functions yields results comparable …

Theoretical Studies Of Electron Transfer In Metal Dimers: Xy+→X+Y, Where X, Y=Be, Mg, Ca, Zn, Cd, Robert J. Cave, David V. Baxter, William A. Goddard Iii, John D. Baldeschwieler

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The electronic matrix element responsible for electron exchange in a series of metal dimers was calculated using ab initio wave functions. The distance dependence is approximately exponential for a large range of internuclear separations. A localized description, where the two nonorthogonal structures characterizing the electron localized at the left and right sites are each obtained self‐consistently, is found to provide the best description of the electron exchange process. We find that Gaussian basis sets are capable of predicting the expected exponential decay of the electronic interactions even at quite large internuclear distances.