  The relaxation dynamics of glass-forming systems shows a multitude of
features that are absent in normal liquids, such as non-exponential relaxation
and a strong temperature-dependence of the relaxation time. Connecting these
dynamic properties to the microscopic structure of the system is very
challenging because of the presence of the structural disorder. Here we use
computer simulations of a metallic glass-former to establish such a connection.
By probing the temperature and wave-vector dependence of the intermediate
scattering function we find that the relaxation dynamics of the glassy melt is
directly related to the local arrangement of icosahedral structures: Isolated
icosahedra give rise to a flow-like stretched exponential relaxation whereas
clusters of icosahedra lead to a compressed exponential relaxation that is
reminiscent to the one found in a solid. Our results show that in metallic
glass-formers these two types of relaxation processes can coexist and give rise
to a surprisingly complex dynamics, insight that will allow to get a deeper
understanding of the outstanding mechanical properties of metallic glasses.
