Research On The Relationship Between Microstructure And Dynamic Properties Of Lennard-Jones Supercooled Liquids

The microscopic dynamical characteristics of supercooled liquids are more complex comparing with high temperature liquids.The β-relaxation caused by slow dynamics can lead to dynamic heterogeneities of the system.In this paper,the structural and dynamical characteristics of supercooled liquids are studied by means of Molecular Dynamics simulations.The static structural features of supercooled liquids are similar to that of high temperature melts.Supercooled liquids are short-ranged order structures,but the pair distribution functions of supercooled liquids split at the second peaks.Within short ranges,the correlation between atoms increases with the decreasing of temperatures.The dynamic characteristics of supercooled liquids are essentially different from that of high temperature liquids.Mean square displacements and self intermediate scattering functions of supercooled liquids show plateaus in the medium-time range.The lower the temperature is,the longer the duration of the plateau persists.Non-Gaussian parameter increases exponentially in theβ-relaxation stage,revealing that the atomic displacement distribution gradually starts to break away from the Gaussian distribution,showing dynamic heterogeneities.In the high temperature regions,the values of four point correlation functions are close to zero,which indicates that atoms participating in the collective motion at high temperatures are fewer and the system is uniform.With the decrease of temperatures,the number of collective atoms increases,and the dynamic heterogeneities of the system occur.The results show that the dynamic heterogeneities caused by temperatures is the essential factor for the glass transition of supercooled liquids.In order to further reveal the microscopic mechanism of dynamic heterogeneities in supercooled liquids,an investigation shows that atomic hopping takes place in supercooled liquids.There are strong short-range correlations between the hopping atoms.Through the analysis of dynamic characteristics,it is found that the correlation between the initial position and hopping atom increases with the increasing of motion time.The first peak of pair distribution function of caged atoms around hopping atoms indicates that the caged atoms have short-ranged order structures.By analyzing the dynamic characteristics of caged atoms,it is found that the correlation between the initial position and hopping atom weakens with the increase of time.The hopping atoms can make the caged atoms move cooperatively,resulting in the dynamic heterogeneities.The influence of dynamic heterogeneities on nucleation is further explored.By identifying HCP-like and FCC-like atoms in the system,our results show that the lower the temperature is,the more the number of crystalline-like atoms is.Other atoms clustering around HCP-like or FCC-like atoms form short-ranged order structures.There are shortrange correlations between crystalline-like clusters.We found two mechanisms that HCPlike and FCC-like atoms are formed: the participation of external atoms and the recombination of neighboring atoms.Most of the HCP-like and FCC-like atoms are formed by the recombination of neighboring atoms,which reflects the importance of dynamic heterogeneities for the nucleation.