Dislocations and vacancies in two-dimensional mixed crystals of spheres and dimers
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Gerbode, S.J.; Ong, D.C.; Liddell, C.M.; Cohen, I.
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 introduces finite dislocation cages, suggesting that glassy dynamics are present for any mixed crystal. However, we have also identified a vacancy-mediated uncaging mechanism for releasing dislocations from their cages. This mechanism is dependent on vacancy diffusion, which slows by orders of magnitude as the dimer concentration is increased. We propose that in mixed crystals with low dimer concentrations vacancy diffusion is fast enough to uncage dislocations and delay the onset of glassy dislocation dynamics.
We thank Jim Sethna, Stefano Zapperi, Fernando Escobedo, Umang Agarwal, Carl Franck, Stephanie Lee, Erin Riley, and the Cohen group for helpful discussions. This research was supported in part by the Department of Energy, Basic Energy Sciences, Grant No. ER46517 fabrication of colloidal assemblies and manipulation with optical trap and in part by Award No. KUS-C1-018-02 from King Abdullah University of Science and Technology KAUST .
Physical Review E
Colloidal crystals; Vacancey motion
Previously Published As
Phys Rev E Stat Nonlin Soft Matter Phys. 2010 Oct; 82(4 Pt 1):041404. Epub 2010 Oct 15