Using numerical simulations, an extensive finite-size analysis was made of the transverse diffusion coefficient in a sheared two-dimensional amorphous solid over a broad range of strain rates at temperatures up to the supercooled liquid regime. Direct qualitative evidence for the persistence of correlations was thus obtained between elementary plastic events up to the vicinity of the glass transition temperature Tg. A quantitative analysis of the data, combined with a previous study of the T and γ dependences of the macroscopic stress led to the conclusion that the average avalanche size remained essentially unaffected by temperatures of up to 75% of Tg. Thus, the primary outcome was that, in a two-dimensional sheared system at finite temperatures and strain rates, correlations between elementary plastic events persisted up to the vicinity of the glass transition. They showed up as a stronger-than-log size-dependence of the diffusion coefficient. Moreover, a collapse of re-scaled diffusion data above a crossover point, valid up to the glass transition, led to the conclusion that avalanches were unaffected by temperature in the shear-controlled regime. This offered further support to a previous conclusion, based upon an analysis of macroscopic rheology that, at up to 75% of Tg, the average avalanche size remained essentially unaffected by temperature. For all the strain rates which were studied, and for most temperatures, the system was in the shear-controlled diffusive regime. It was only upon approaching Tg that the effects of mechanical and thermal noise could no longer be unravelled. It was then likely that, in this regime, thermal noise gradually destroyed any correlations. A thermal enhancement of diffusion, found at lower strain rates, could reasonably be attributed to the rejuvenation of local configurations due to plastic activity which permanently fed additional thermal relaxation. However, the conclusions were restricted to two-dimensional systems. Nevertheless, it was believed that some of them should carry over to three dimensions. Indeed, there was a striking similarity between the present crossover between strain-controlled and temperature-controlled regimes, found in two dimensions, and the curve that delimited inhomogeneous and homogeneous flow regimes in metallic glasses. The overall observations strongly suggested that, in three dimensions as well as in two dimensions, it was the long-range elastic couplings responsible for avalanches which remained prevalent over the whole range of the strain-controlled regime.

Robustness of Avalanche Dynamics in Sheared Amorphous Solids as Probed by Transverse Diffusion. J.Chattoraj, C.Caroli, A.Lemaître: Physical Review E, 2011, 84[1], 011501