Correlations and critical behavior of the q-model
  The q-model is a random walk model used to describe the flow of stress in a stationary granular medium. Here we derive the exact horizontal and vertical correlation functions for the q-model in two dimensions. We show that close to a critical point identified in earlier work these correlation functions have a universal scaling form reminiscent of thermodynamic critical phenomena. We determine the form of the universal scaling function and the associated critical exponents $\nu$ and $z$. 
Alexander V. St. John, Harsh Mathur
cond-mat.dis-nn cond-mat.mes-hall cond-mat.stat-mech