News Source & Photographer: School of Physics

Editor: Wang Lirong. Translator: Huang Yuxuan, News Agency of BIT

Recently, Professor Li Yanwei and Professor Yao Yugui, from School of Physics, BIT, have collaborated with Professor Massimo Pica Ciamarra from School of Mathematical Physics, Nanyang Technological University, Singapore, and made important progress in two-dimensional melting research. The relevant results were published in Physical Review Letters, a top journal in physics.

In three-dimensional systems, the crystal-liquid transition is usually a first-order phase transition without intermediate phase, while the two-dimensional crystal-to-liquid transition is more complex and interesting. According to the theory of KTHNY, in two-dimensional systems, there may be an intermediate hexagonal phase between the crystal and the liquid: recent studies have shown that while the transition from crystal to hexagonal phase is continuous, the transition from hexagonal phase to liquid can be either continuous or first-order. The melting process is also accompanied by the evolution of system defects, such as dislocations in crystals often appear in pairs; pairwise dislocations dissociate into isolated dislocations in the hexagonal phase; isolated dislocations in liquids can be further dissociated into disclination. Up to now, three different two-dimensional melting mechanisms have been discovered, namely, the crystal-liquid primary phase transition mechanism without intermediate phase, the mechanism of two successive crystal-hexagonal phase and hexagonal phase-liquid continuous phase transition predicted by KTHNY theory and mixing mechanism of crystal-hexagonal phase continuous phase transition and hexagonal phase-liquid primary phase transition. Many factors can affect the two-dimensional melting mechanism, such as particle shape. Research has found that one-component regular hexagonal systems with hard interactions follow the continuous phase transition mechanism, circular systems follow the mixing mechanism, and regular pentagonal systems follow the primary phase transition mechanism.

Previous studies have often focused on single-component systems, and the melting behavior of multi-component systems is more complex, such as the ratio relying on multiple components, and how to select and transition the melting mechanism to one-component. Is it possible to mix two component particles while following a different melting mechanism than the single component? Is it possible to obtain low eutectic mixtures below the one-component melting temperature or above the one-component melting density in hard particle systems? Are topological defects as closely related to phase behavior as one-component systems?

Figure 1. Distribution of defects (red and blue) in the solid phase (left), hexagonal phase (middle), solid-liquid coexistence phase (right) in a circular-regular pentagonal two-component hybrid system. Red, green, and blue represent particles with 5, 6, and 7 neighbors, respectively.

Focusing on the above scientific problems, the team designed two-component and three-component hybrid systems of circular, regular hexagonal and regular pentagonal shapes of the same size, and studied the phase behavior and defect evolution of the system under different proportions based on molecular dynamics simulation. Due to the homogeneity of size, the mixing of particles of different shapes is homogeneous (Figure 1), and there is no phase separation or glass transition process.

Figure 2. The dependence law of (a) melting phase diagram and (b) defect density with ratio of different two-component systems

In the two-component system (top column of Figure 2), with mixing circular and regular hexagon particles and mixing regular pentagonal and regular hexagonal particles, the melting mechanism of the system transitions from one one-component mechanism to another. By mixing regular pentagonal and round particles, we find that the mixed system can follow the KTHNY continuous phase transition mechanism, which is different from the two-component mechanism, and this mixed system is a low eutectic mixture system. We further study the evolution of topological defects (the number of neighbors of particles other than 6) and find that when the defect concentrations are greater than 0.046 and greater than 0.123 (lower column of Figure 2), the solid phase and hexagonal phase are instable, which provides a criterion for determining the phase state by the defect concentration. We further established a ternary melting phase diagram and found that the KTHNY phase transition mechanism is always between the primary phase transition mechanism and the mixing mechanism. This work provides useful insights for understanding the two-dimensional melting mechanism.

Figure 3. Diagram of the melted phase of a three-component system

The relevant work was published in Physical Review Letters, with Li Yanwei, a researcher from the School of Physics, BIT, as the first author and Professor Yao Yugui from the School of Physics, BIT, as the corresponding author, and Massimo Pica Ciamarra from Nanyang Technological University, Singapore, as co-corresponding authors of the paper. The first unit of this work is the School of Physics of BIT, and the work is supported by the National Natural Science Foundation of China and the BIT Young Scholars Academic Start-up Program.