The Gardner correlation length scale in glasses

The Gardner length scale $\xi$ is the correlation length in the vicinity of the Gardner transition, which is an avoided transition in glasses where the phase space of the glassy phase fractures into smaller sub-basins on experimental time scales. We argue that $\xi$ grows like $\sim \sqrt{B_{\infty}/G_{\infty}}$, where $B_{\infty}$ is the bulk modulus and $G_{\infty}$ is the shear modulus, both measured in the high-frequency limit of the glassy state. We suggest that $\xi$ might be inferred from stress-stress correlation functions, which is more practical for experimental investigation than studying two copies of the system, which can only be done in numerical simulations. Our arguments are illustrated by explicit calculations for a system of disks moving in a narrow channel, which is solved exactly by transfer matrix techniques.