Abstract
Molecular dynamics computer simulations of collective excitations in a system of hard disks confined to a narrow channel of the specific width, that resembles 2D triangular lattice at disk close packing, are performed. We found that transverse excitations, which for hard-disk system are absent in the limit of 1D and are of acoustic nature in the limit of 2D, in the case of q1D hard-disk system emerge in the form of transverse optical excitations and could be considered as a tool to detect the structural transition to a zigzag ordering. By analyzing density evolution of longitudinal static structure factor and pair distribution function we have shown that driving force of zigzag ordering is caging phenomenon that in the case of hard-disk system is governed by excluded volume interaction with first and second neighbors and is of entropic origin.
Highlights
The interest in a quasi-one-dimensional (q1D) hard-core fluid has both basic [1,2,3,4,5,6] and applied [7,8,9,10] aspects
The freezing-melting transition [15] and collective dynamics [1,16] in 2D hard-core systems are related to the caging phenomenon [17], which is a hindrance of particle motion by its nearest neighbors (NN) [18]
We resort to the q1D hard disk (HD) system confined to a pore to get insight into the role of caging-uncaging events
Summary
The interest in a quasi-one-dimensional (q1D) hard-core fluid has both basic [1,2,3,4,5,6] and applied [7,8,9,10] aspects. A solid-to-fluid transition is a global phenomenon attributed to the entire body, but in a 2D crystals it starts from local emergence of bounded defect pairs [21]. This might have similarity to the melting in a q1D HD system. The density of defects is determined by their core energy via Boltzmann’s factor, which is irrelevant to HD systems where possible defects have a purely entropic origin Can such entropy-driven local uncaging, resembling thermal excitation of bounded pairs in the Kosterlitz-Thouless scenario, be effective in the zigzag melting? As density decreases, both simulation and theory predict an emergence of progressively larger number of uncaged disk pairs. This picture is in line with a continuous Kosterlitz-Thouless-type transition
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