Atomic Cooperativity in Metallic Glasses and Liquids

Liquids and glasses are condensed matter with density comparable to those of crystalline solids. Thus, atoms are not free to move, and they are strongly correlated with each other in space and time. In the glassy state atoms are confined and caged by neighbors. Understanding and specifying the nature of dynamic heterogeneity and cooperativity in terms of such correlations has long been the focus of glass physics. In this talk I review the scales of dynamic cooperativity in space and time and present a new view of the phenomena based on the density wave theory. Starting with Adam and Gibbs, historically cooperative motions were thought to occur around low-density areas, such as free-volume. But that thinking reflects a bias toward the hard-sphere models, whereas atoms are not hard spheres. Local density can deviate from the average both in positive and negative directions. Then, a better way to characterize the state of liquid and glass is through density fluctuations, decomposed into density waves. I show when atomic interactions are introduced to a gas, density waves are created in all directions. But density waves are unstable against local thermal and quantum fluctuations and become spatially limited to form the medium-range order (MRO). Many phenomena, including mechanical deformation, viscosity and relaxation, can be described in terms of the interplay among density waves, local topology of atomic connectivity and local elasticity.