Temperature Induced Randomness in Crystalline Dielectrics

Understanding the dynamics of atomic and molecular vibrations is critically important to understanding thermal transport behavior of nanostructured and bulk materials. Capturing these dynamics using atomistic scale computational tools, like molecular dynamics, can be challenging due to the strict limitations on system size. The dynamics of vibrations in amorphous materials, however, can be modeled using approaches such as random matrix theory, which have the capability of modeling materials at sizes well beyond that of molecular dynamics. The random matrix method applies to amorphous materials due to the random nature of vibrations that stem from the long-range disordered structure of amorphous materials. This work evaluates the possibility of finding similar random behavior in crystalline materials. We hypothesize that the system temperature, which is related to the system kinetic energy, can be treated as a source of randomness in the system and we evaluate this hypothesis by conducting molecular dynamics simulations of Lennard-Jones argon and comparing the system behavior using the traditional potential function to one in which randomness has been introduced through the application of Brownian dynamics (a random force term). Our results show changes in the lifetime of the vibrational modes between the traditional potential and the traditional potential with the addition of random perturbations. The difference between the lifetime of the modes in an argon system with and without randomness is reduced at higher temperatures and they become indistinguishable at approximately half of the Debye temperature. This observation is supported by observing local non-coherent modes at higher temperatures. Observed similarities in vibrational behavior between crystalline and amorphous materials will enable the adoption of more computationally efficient methods for the study of crystalline materials.