Molecular Dynamics Simulation of Coherent Phonon Generation and Its Spectral Characterization in a Nanoribbon upon Localized Pulse Heating

Abstract

The generation and thermal transport of coherent phonons during instantaneous pulse heating in the presence of diffusion is studied by a molecular dynamics (MD) method. Coherent phonon formation and propagation characteristics are obtained and compared for different shapes of the heating pulse, such as a half-period square, a Gaussian, and a triangle, using the Lennard-Jones (LJ) nanoribbon model. Heating energy exceeding the equilibrium energy distribution of a heated region relaxes by emitting a train of (3 to 5) coherent phonons. As shown in the MD model, the equations of heat flux can resolve coherent phonon motion with high resolution when flux through the boundaries is evaluated with sampling regions of the same size as a single phonon vibration period in the direction of propagation. In the presence of diffusion, the dependence of the generation and decay of phonons on the energy density of the heating pulse is studied for different heating times of the nanoribbon sample. Heating pulses of different duration with a Gaussian profile lead to a higher percentage of heating energy being converted into coherent phonons relative to other pulse shapes. The number of generated phonons and their amplitudes are shown to vary with the pulse duration and shape owing to differences in the energy density of the heating pulses. In the phonon propagation sampling regions, the density of states (DOS) is used to identify coherent phonon frequencies, which are shown to correspond, in terms of the number of identified phonons, to the shape of the thermal envelope for the different pulse shapes and heating times of the nanoribbon sample.