Parts | Days | Selection | Search | Downloads | Help
DY: Dynamik und Statistische Physik
DY 10: Critical Phenomena
DY 10.4: Talk
Monday, March 8, 2004, 10:30–10:45, H2
Critical Dynamics: theory for time-dependent intensity correlations measured by X-ray microdiffraction — •Klaus Mecke1, Cristian Mocuta2, and Harald Reichert1 — 1MPI für Metallforschung, D-70569 Stuttgart, Germany — 2ESRF, F-38043 Grenoble, France
We studied the dynamics of critical fluctuations in Fe3Al, which exhibits an order-disorder phase transition (DO3-B2, TC =780 K). In the experiments we used a well focused beam to probe a sub-micron sized single crystalline sample, so that the size of fluctuating domains is large compared to the probed volume due to the diverging correlation length close to TC. Pronounced effects on the time-course of the intensity of a superstructure peak (characteristic for the chemical order) were measured. Starting from a stochastic Langevin equation for the fluctuations of a non-conserved order-parameter (model A) the four-point structure function can be calculated and directly related to the measured time-dependent intensity-intensity correlation function. The analytical expression fits very well the experimental data sets fixing the correlation time τ as only free parameter of the theory. We find a strong dependence on temperature, in particular an increase of τ close to the critical point. The agreement of theory and experiments - even for various spatial and time resolutions - demonstrate the potential of partially coherent x-ray microbeams for the direct observation of fluctuation phenomena at critical phase transition.