SPATIALLY HETEROGENEOUS DYNAMICS IN A 2-DIMENSIONAL GLASS-FORMING BINARY MIXTURE

A quantitative analysis is presented of the increasing degree of dynam ical correlations with decreasing temperature in a fragile glass-forme r consisting of a two-dimensional binary mixture of soft discs with a diameter ratio of sigma(2)/sigma(1) = 1.4. The analysis involves a stu dy of the spatial distribution of local relaxation times, defined as t he time taken for each particle to first travel a distance r from its initial position. For the binary mixture, a temperature-independent op timum value of r approximate to sigma(1) is found to maximize the spat ial segregation of particles into different kinetic domains. The regio ns of 'fast' and 'slow' particles grow in size-as the system is cooled , indicating an increasing degree of cooperativity in the particle dyn amics. A measure of the linear dimensions of these clusters is provide d. It is shown that only 'slow' subsets of particles are caged on inte rmediate timescales and that the lifetime of these slow domains increa ses dramatically with decreasing temperature in the supercooled mixtur e. A substantial decay in the incoherent scattering functions can stil l be accomplished, however, on these timescales, despite the relative immobility of a significant fraction of the system: A further observat ion is a change in the manner in which relaxation progresses throughou t the system with cooling. At high temperatures the initially fast rel axing sites are randomly distributed throughout the system, whereas at low temperatures they tend to be clumped together. This subsequently results in a less homogeneous progression of relaxation at the lower t emperatures, since relaxation proceeds primarily by radiating outwards from existing fast centres.