Spectral Molecular Dynamic Analysis of Coherent Phonon Propagation in Nanoribbon after Pulse Heating

Abstract

Thermal transport of coherent phonons at a few picosecond pulse heating is studied by molecular dynamics method in the presence of diffusion. The presence of the acoustic and optical phonons at the high density heating of nanostructures has been confirmed theoretically and experimentally in the absence and presence of diffusion. In the present study, coherent phonon spectral characteristics are compared for different shapes of the heating pulse, half-pulse square, Gaussian, and triangle at the time of propagation, in the Lennard-Jones (LJ) nanoribbon model for emitted train (3 to 5) of coherent phonons. In order to analyze the spectral contributions of individual phonons, in the molecular dynamic (MD) model, density of states (DOS) at propagation subregions is utilized for identification of coherent phonon spectra for the different pulse shapes and heating times in the nanoribbon sample. The MD equations can resolve wave motion for coherent phonons over sampling subregions that correspond in size (several atomic layers) to a single phonon vibration period. However, the definition of DOS does not distinguish the spectral characteristics of each of individual phonons as well as separates the diffusional spectrum from the coherent phonons one. In the presence of diffusion, spectrum of the first generated phonon is studied by varying the coherent phonon position inside of time interval that is used for the quasi-equilibrium DOSes defined over one of the central regions. The identification of diffusional frequencies for the nanoribbon permit to extract the coherent phonon frequencies characteristic for the propagation in the nanoribbon studied. In the case of propagating coherent phonons, it was shown that the frequencies of some phonons can be identified. Increase of the temporal resolution for the DOS calculation is shown to be critical for separation of the diffusion process spectrum and the one of phonons at the same area of calculation.