Solid-to-molecular-orientational-hexatic melting induced by local environment determined defect proliferations

Two-dimensional (2D) melting is a fundamental research topic in condensed matter physics, which can also provide guidance on fabricating new functional materials. Nevertheless, our understanding of 2D melting is still far from being complete due to existence of possible complicate transition mechanisms and absence of effective analysis methods. Here, using Monte Carlo simulations, we investigate 2D melting of 60° rhombs which melt from two different surface-fully-coverable crystals, a complex hexagonal crystal (cHX) whose primitive cell contains three rhombs, and a simple rhombic crystal (RB) whose primitive cell contains one rhomb. The melting of both crystals shows a sequence of solid, hexatic in molecular orientation (H^mo), and isotropic phases which obey the Berezinskii-Kosterlitz-Thouless-Halperin-Nelson-Young (BKTHNY) theory. However, local polymorphic configuration (LPC) based analysis reveals different melting mechanisms: the cHX-H^mo transition is driven by the proliferation of point-like defects during which defect-associated LPCs are generated sequentially, whereas the RB-H^mo transition is driven by line defects where defect-associated LPCs are generated simultaneously. These differences result in the observed different solid-H^mo transition points which are φ_A=0.812 for the cHX-H^mo and φ_A=0.828 for the RB-H^mo. Our work will shed light on the initial-crystal-dependence of 2D melting behavior.