Particle rearrangements during transitions between local minima of the potential energy landscape of a binary Lennard-Jones liquid

The potential energy landscape (PEL) of binary Lennard-Jones (BLJ) mixtures exhibits local minima, or inherent structures (IS), which are organized into metabasins (MBs). We study the particle rearrangements related to transitions between both successive IS and successive MB for a small 80:20 BLJ system near the mode-coupling temperature T-MCT. The analysis includes the displacements of individual particles, the localization of the rearrangements, and the relevance of string-like motion. We find that the particle rearrangements during IS and MB transitions do not change significantly at T-MCT. In particular, an onset of single particle hopping on the length scale of the interparticle distance is not observed. Further, it is demonstrated that IS and MB dynamics are spatially heterogeneous and facilitated by string-like motion. To investigate the mechanism of string-like motion, we follow the particle rearrangements during suitable sequences of IS transitions. We find that most strings observed after a series of transitions do not move coherently during a single transition, but subunits of different sizes are active at different times. Several findings suggest that, though string-like motion is of comparable relevance when the system explores a MB and when it moves from one MB to another, the occurrence of a successful string enables the system to exit a MB. Moreover, we show that the particle rearrangements during two consecutive MB transitions are basically uncorrelated. In particular, different groups of particles are highly mobile. We further find the positions of strings during successive MB transitions weakly but positively correlated, supporting the idea of dynamic facilitation. Finally, the relation between the features of the potential energy landscape and the relaxation processes in supercooled liquids is discussed. (C) 2004 American Institute of Physics.