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

We calculated a four-point correlation function G(4)((k) over right arrow, (r) over right arrow; t) and the corresponding structure factor S-4((k) over right arrow, (q) over right arrow; t) for a model glass-forming binary mixture. These functions measure the spatial correlations of the relaxation of different particles. We found that these four-point functions are anisotropic and depend on the angle between vectors (k) over right arrow and (q) over right arrow (or (q) over right arrow). 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, tau(alpha). At the lowest temperatures S-4((k) over right arrow, (q) over right arrow; tau(alpha)) is strongly anisotropic even for the smallest wavevector q accessible in our simulation.