Anisotropic spatially heterogeneous dynamics in a model glass-forming binary mixture

We used computer simulations to calculate, for a model binary mixture, a four-point correlation function which measures the spatial correlations of the relaxation of different particles and the corresponding structure factor. We found that these four-point functions are anisotropic. The anisotropy is the strongest for times somewhat longer than the $\beta$ relaxation time, but it is quite pronounced even for times comparable to the $\alpha$ relaxation time. At the lowest temperatures the four-point structure factor is strongly anisotropic even for the smallest wavevector $q$ accessible in our simulation.