Shape anisotropy controls 2D melting pathway

Abstract

The melting of two-dimensional systems is a fundamental challenge in condensed matter physics, where topological defects and thermal fluctuations play a key role. This work uses molecular dynamics simulations to investigate the melting of particles interacting via the Gay–Berne potential in the weak anisotropy regime (1.0 ≤ k ≤ 1.2). We demonstrate that the melting mechanism depends critically on the particle aspect ratio. For weak anisotropy (k < 1.15), the system follows a hybrid Bernard–Krauth scenario, featuring a continuous crystal-to-hexatic transition, followed by a first-order hexatic-to-isotropic liquid transition. At k ≥ 1.15, the system switches to the full Berezinskii–Kosterlitz–Thouless–Halperin–Nelson–Young scenario with two continuous Berezinskii–Kosterlitz–Thouless transitions. Introducing binary mixtures of particles with different anisotropies suppresses the first-order transition, stabilizing the continuous melting pathway. Therefore, weak shape anisotropy serves as a fundamental switching parameter governing the universal melting behavior of two-dimensional systems.