Alladi Ramakrishnan Hall

Entropy driven transitions: A case of hard rods on a cubic lattice

Vigneshwar Narayanan

IMSC

Entropy transitions are those transitions which are primarily driven by the geometry of the configurations, unlike those driven by temperature. In effect, these are simpler than normal transitions as there is no internal energy involved. These are the simplest systems for which, transitions occur without any attractive interactions.

We study the different phases of a system of monodispersed hard rods of length k on a cubic lattice. This is the simplest system involving anisotropic particles to exhibit entropy driven transitions in 3D. This system shows a rich variety of phases. When k is less than or equal to 4, the system is disordered at all densities. For k=5,6, we find a single density-driven transition from a disordered phase to high density layered-disordered phase in which the density of rods of one orientation is strongly suppressed, breaking the system into weakly coupled layers.When k is greater than or equal to 7, three density driven transitions are observed numerically: isotropic to nematic to layered-nematic to layered-disordered. In the layered-nematic phase, the system breaks up into layers, with nematic order in each layer, but very weak correlation between the ordering directions of different layers. We argue that the layered-nematic phase is a finite-size effect, and in the thermodynamic limit, the nematic phase will have higher entropy per site. We expect the systems of
rods in four and higher dimensions will have a qualitatively similar phase diagram.