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-fullycoverable 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(Hmo),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-Hmotransition is driven by the proliferation of point-like defects during which defect-associated LPCs are generated sequentially,whereas the RB-Hmotransition is driven by line defects where defect-associated LPCs are generated simultaneously.These differences result in the observed different solid-Hmotransition points which areφA=0.812 for the cHX-HmoandφA=0.828 for the RB-Hmo.Our work will shed light on the initial-crystal-dependence of 2D melting behavior.

Phase behavior of rotationally asymmetric Brownian kites containing 90°internal angles

Previous Monte Carlo simulations have shown that ordered tetratic phases can emerge in a dense two-dimensional Brownian system of rotationally asymmetric hard kites having 90°internal angles.However,there have been no experimental investigations yet to compare with these simulation results.Here,we have fabricated two types of micron-sized kites having internal angles of 72°-90°-108°-90°and 72°-99°-90°-99°,respectively,and we have experimentally studied their phase behavior in two-dimensional systems.Interestingly and in contrast to the Monte Carlo simulations,the experimental results show a phase sequence of isotropic fluid-hexagonal rotator crystal-square crystal as the area fractionφA increases for both types of kites.The observed square crystal displays not only a quasi-long-range translational order but also(quasi-)long-range 4-fold bond-and molecular-orientational order;these characteristics confirm that tetratic order can emerge even in dense Brownian systems of rotationally asymmetric particles.A model based on local polymorphic configurations(LPCs)is proposed to understand the origin of the square lattice order in these dense kite systems.The results in this study provide a new route to realize custom-designed self-assembly of colloids by controlling LPCs.

The cooperative migration dynamics of particles correlates to the nature of hexatic-isotropic phase transition in 2D systems of corner-rounded hexagons

In the two-dimensional(2D)melting transition of colloidal systems,the hexatic-isotropic(H-I)transition can be either first-order or continuous.However,how particle dynamics differs at the single-particle level during these two different melting transitions remains to be disclosed.In this work,by Brownian dynamics(BD)simulations,we have systematically studied the dynamic behavior of corner-rounded hexagons during the H-I transition,for a range of corner-roundness𝜁=0.40 to 0.99 that covers the crossover from the continuous to first-order nature of H-I transition.The results show that hexagons with𝜁≤0.5 display a continuous H-I transition,whereas those with𝜁≥0.6 demonstrate a first-order H-I transition.Dynamic analysis shows different evolution pathways of the dominant cluster formed by migrating particles,which results in a droplet-like cluster structure for𝜁=0.40 hexagons and a tree-like cluster structure for𝜁=0.99 hexagons.Further investigations on the hopping activities of particles suggest a cooperative origin of migrating clusters.Our work provides a new aspect to understand the dependence of the nature of H-I transition on the roundness of hexagons through particle dynamic behavior.