Dresden 2009 – wissenschaftliches Programm

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CPP: Fachverband Chemische Physik und Polymerphysik

CPP 24: Colloids

CPP 24.1: Vortrag

Mittwoch, 25. März 2009, 14:00–14:15, ZEU 160

Forbidden Symmetries in 2D Colloidal Systems — •Jules Mikhael, Sebastian Rausch, Laurent Helden, and Clemens Bechinger — 2. Physikalisches Institut, Universität Stuttgart, Pfaffenwaldring 57, 70569 Stuttgart

Until 1984, it was unanimously established that rotational symmetries like 5-, 7-, 8-fold or higher are forbidden to ordered matter. However some metal alloys, polymers or micelles have defied these crystallographic rules and self-organized into so-called quasicrystals. Only four rotational symmetries mainly the 5-, 8-, 10- and 12-fold were reported although geometrical models predict any n-fold symmetry (where n is an integer number). Why? Are there forbidden symmetries even to quasicrystals?

Here we artificially create one of the symmetries, never observed in nature, by interfering seven coherent laser beams. Due to optical forces, the generated pattern acts as a template for a colloidal monolayer whose phase behavior is studied in real space. Our results demonstrate that for interference patterns of 7 laser beams the colloids organize in large domains with strict periodic, i.e. crystalline, order. This is in contrast to interference patterns comprised of 5 laser beams where the colloids adopt the quasicrystalline order imposed by the substrate[1]. Based on the substrate potential depth distribution of interference patterns with different rotational symmetries, we provide a possible explanation why 5-, 8-, and 10-fold symmetries can occur in self organized systems while 7-, 9-, 11-fold do not.

[1] Mikhael, Roth, Helden & Bechinger, Nature 454, 501 (2008).