Experimental Evidence that Aging is a Log-Poisson Process

It is by now widely recognized that the aging process, which ever so slowly relaxes a complex disordered material after a quench into a glassy state, is driven by activated processes. Especially the process of full aging following a hard quench is characterized universally, for a wide class of materials, by logarithmic evolution of state variables and a power-law decay of two-time correlation functions that collapse only for the ratio of those times, instead of their difference, as time-translational invariance in equilibrium would demand. Yet, many theories often cited to describe such a non-equilibrium process can be shown to miss pertinent aspects that are inherent to aging experiments. A case in point is provided by recent experiments on crumpling sheets of mylar loaded by a weight whose acoustic emissions are measured while the material buckles. Analyzing those local avalanches on various length scales shows [1] that crumpling is a log-Poisson process activated by increasingly rare record-sized fluctuations in a slowly stiffening material, an effect quite similar to recent observations for various other glasses [2].