Equilibrium versus non-equilibrium surface diffusion measurements

Abstract

Surface diffusion coefficients under equilibrium conditions can be measured from the decay of the autocorrelation function of concentration fluctuations monitored within a probe area centered on the plane. Such a method is implemented with the use of the Scanning Tunneling Microscope by recording tunneling current fluctuations in real time. The form and decay constant of the autocorrelation function can be varied as a function of coverage, temperature (i.e., adatom interactions), probe area size (by varying the tip- surface separation) and electric field strength. Experimental results will be presented and compared to theoretical expectations based on Monte Carlo simulations on model systems. On the other hand non-equilibrium experiments are carried out during ordering kinetics (i.e., the evolution of a system from an initial disordered to a final ordered state after a temperature quench) to determine the growth laws. Time dependent non- equilibrium diffusion coefficients are defined under these conditions and their relation to equilibrium diffusion coefficients is clarified in terms of the adsorbate-adsorbate interactions.