Yield stresses and flow curves in metallic glass formers and granular systems

We discuss the concept of a glass transition line in the temperature-shear-stress plane in the context of recent simulation data for a metallic melt and dense-packed granular systems. Analyzing these data within a schematic model of the mode-coupling theory for dense glass formers under shear, values for the critical dynamic yield stress (the stress resulting in the limit of arbitrarily slow shear, at the glass transition) are estimated. We discuss two possible scenarios, that of a continuous rise in the dynamic yield stress at the transition, and that of a discontinuous transition, and discuss the data range that needs to be covered to decide between the two cases. A connection is made to the two commonly drawn versions of the jamming diagram, one convex and one concave regarding to the shape of the solid region.