Inherent structures, fragility, and jamming: insights from quasi-one-dimensional hard disks

We study a quasi-one-dimensional system of hard disks confined between hard lines to explore the relationship between the inherent structure landscape, the thermodynamics, and the dynamics of the fluid. The transfer matrix method is used to obtain an exact description of the landscape, equation of state, and provide a mapping of configurations of the equilibrium fluid to their local jammed structures. This allows us to follow how the system samples the landscape as a function of occupied volume fraction ϕ. Configurations of the ideal gas map to the maximum in the distribution of inherent structures, with a jamming volume fraction ϕ(J)(*), and sample more dense basins with increasing ϕ. This suggests jammed states with a density below ϕ(J)(*) are inaccessible from the equilibrium fluid. The configurational entropy of the fluid decreases rapidly at intermediate ϕ before plateauing at a low value and going to zero as the most dense packing is approached. This leads to the appearance of a maximum in both the isobaric heat capacity and the inherent structure pressure. We also show that the system exhibits a crossover from fragile to strong fluid behavior, located at the heat capacity maximum. Structural relaxation in the fragile fluid is shown to be controlled by the presence of high order saddle points caused by neighboring defects that are unstable with respect to jamming and spontaneously rearrange to form a stable local environment. In the strong fluid, the defect concentration is low so that defects do not interact and relaxation occurs through the hopping of isolated defects between stable local packing environments.