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Light-Induced Phase Transitions


Increasing particle fluctuations lead typically to a higher degree of disorder i.e. to melting. Although this intuitive relation between melting and fluctuations is widely accepted among physicists, fluctuations can also enhance order under certain conditions. We study this – at the first glance paradoxical effect by superimposing optical interference patterns to two-dimensional colloidal systems. Under the influence of light forces the dielectric spheres are driven into regions of highest light intensity I. When I is increased, particle fluctuations perpendicular to the laser lines are decreased and a transition from a disordered into an ordered state occurs. However, upon further increase of the laser intensity, i.e. a reduction of fluctuations perpendicular to the light potential, the crystal is found to remelt (laser-induced-melting) to a 1D liquid. A similar behavior is also observed when three laser beams are used to produce a 2D light potentials. In this case a novel colloidal state, i.e. a colloidal molecular crystal is found.


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