Science / Technology - Colloquium
Monday, January 21, 2008
4:00 PM-5:00 PM
Olin Hall
107
Classical phase transition theory predicts that a homogeneous system is unstable against spontaneous phase-separation into an inhomogeneous structure if the second derivative of its free energy, G, as a function of the order parameter, h0, is negative. However, this prediction a result of the mean-field formalism is expected to fail if the fluctuations in the system order parameter due to thermal fluctuations are larger than the average order parameter, which occurs either when |h0 hsp|/hsp 0 (where hsp is the order parameter such that G(hsp) = 0) or when the dimension of the system is less than six. This prediction is well attested in three-dimensional (3D) systems, where the effects of fluctuations are usually found to be important only when h0 is within a few % from hsp. No experimental study has been performed on 2D systems, where the effects of fluctuations is stronger. Here, we use polystyrene films deposited on oxide coated silicon as a model 2D system to study the effect of fluctuations on the stability of a system for which the order parameter, h0, is the film thickness. We examine the temporal growth of surface capillary waves on this system of films with different h0. For the most unstable film (where h0 is within 98.8% from hsp), the data fits well to the mean-field theory. But as h0 is increased slightly such that h0 is within 97.7% from hsp, the mean-field theory demonstrates marked disagreement with experiment. We verify that the deviations are caused by large-amplitude fluctuations on the film surface due to stochastic thermal noises. Our results thus show that the stability of a low dimensional system can be easily compromised by spontaneous fluctuations, and cannot be predicted at all by the mean-field theory. We thank the support of NSF through project no. DMR 0706096.
Cost: FREE
Sponsored by: WPI Physics Department, Dr. Stephan Koehler
Suggested Audiences: Adult
E-mail:
sak@wpi.edu
Phone: 508-831-5090
Last Modified: January 16, 2008 at 11:37 AM
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