Prestipino Santi, Saija Franz
Dipartimento di Fisica e di Scienze della Terra, Università degli Studi di Messina, Contrada Papardo, I-98166 Messina, Italy.
CNR-IPCF, Viale F. Stagno d'Alcontres 37, I-98158 Messina, Italy.
J Chem Phys. 2014 Nov 14;141(18):184502. doi: 10.1063/1.4901302.
Two-dimensional crystals of classical particles are very peculiar in that melting may occur in two steps, in a continuous fashion, via an intermediate hexatic fluid phase exhibiting quasi-long-range orientational order. On the other hand, three-dimensional spheres repelling each other through a fast-decaying bounded potential of generalized-exponential shape (GEM4 potential) can undergo freezing into cluster crystals, allowing for more that one particle per lattice site. We hereby study the combined effect of low spatial dimensionality and extreme potential softness, by investigating the phase behavior of the two-dimensional (2D) GEM4 system. Using a combination of density-functional theory and numerical free-energy calculations, we show that the 2D GEM4 system displays one ordinary and several cluster triangular-crystal phases, and that only the ordinary crystal first melts into a hexatic phase. Upon heating, the difference between the various cluster crystals fades away, eventually leaving a single undifferentiated cluster phase with a pressure-modulated site occupancy.
经典粒子的二维晶体非常奇特，因为熔化可能分两步以连续方式发生，通过一个呈现准长程取向有序的中间六方相流体相。另一方面，通过广义指数形状的快速衰减有界势（GEM4势）相互排斥的三维球体可以冻结成簇晶体，每个晶格位点允许有多个粒子。我们在此通过研究二维（2D）GEM4系统的相行为，来研究低空间维度和极端势软度的综合影响。通过结合密度泛函理论和数值自由能计算，我们表明二维GEM4系统呈现一个普通的和几个簇三角晶相，并且只有普通晶体首先熔化成六方相。加热时，各种簇晶体之间的差异逐渐消失，最终留下一个具有压力调制位点占有率的单一未分化簇相。
