Seeing through a glass clearly: Experimental observation of the marginal glass phase

It has recently been shown that in the infinite dimension mean field limit there exists a second phase transition within the glassy phase, dividing stable glass and marginal glass. We have developed a technique to experimentally probe these phases of matter using a colloidal glass. We avoid the difficulties inherent in measuring the long time behavior of glasses by instead focusing on the very short time dynamics of the ballistic to caged transition. We track a single tracer particle within a slowly densifying glass and measure the resulting mean squared displacement (MSD). By analyzing the MSD we find three distinct phases at different packing fractions: 1) a sub-diffusive liquid at low densities, 2) a stable glass phase marked by the appearance of a plateau whose magnitude shrinks with increasing density and 3) a marginal glass phase marked by logarithmic growth towards that plateau. This provides the first experimental evidence for the existence of a marginal glass in 3D.