Regensburg 2004 – wissenschaftliches Programm

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DY: Dynamik und Statistische Physik

DY 10: Critical Phenomena

DY 10.4: Vortrag

Montag, 8. März 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, T_C =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 T_C. 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.