Synchronization in scale-free network with asymmetric coupling
  The propensity for synchronization is studied in a complex network of asymmetrically coupled units, where the asymmetry in a given link is determined by the relative age of the involved nodes. In growing scale-free networks synchronization is enhanced when couplings from older to younger nodes are dominant. We describe the requirements for such an effect in a more general context and compare with the situations in non growing random networks with and without a degree ordering. 
Dong-Uk Hwang, Mario Chavez, Andreas Amann, and Stefano Boccaletti