Example of a Break in Symmetry (IMAGE)
Caption
The picture shows ytterbium ions in an ion Coulomb crystal, taken with an EMCCD camera (electron multiplying CCD camera). The ionized atoms fluoresce in the laser light; the distance between the ions is approx. 10µm to 20 µm. (a) Radial symmetry: At a high radial trap enclosure, the ionized atoms line up like a string of pearls. The radial potential felt by the positively charged particles is depicted at the bottom left. (b) Mirror symmetry: If the strength of the radial enclosure changes, the ions become free to avoid each other, and the crystal adopts a new, energetically more favourable structure. Hereby, two energetically equivalent configurations can be chosen from. Areas which do not communicate with each other decide independently of each other on a new orientation. Topological defects occur where two areas which have made different "decisions" collide. The black-and-white images show fluorescing ytterbium ions.
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