Molecular Dynamics Simulation of Heat Transport in a Nanoribbon by the Thermal Wave at Different Durations of Pulse Heating

Abstract

In recent year, progress has been made in the study of ballistic heat flow and phonon scattering by phonon spectroscopy and phonon-imaging techniques. Regarding the femtosecond laser application to nanostructures, phonon generation in nanoscale electronics is the focus of interest in the investigation of the mechanism of thermal wave formation at different heating pulses and conditions for heat flux propagation in nanostructures. We test an atomic model of thermal transport in a nanoribbon after a few picosecond pulse heating that leads to the simultaneous presence of two modes, namely, coherent phonons and diffusion, by molecular dynamics (MD) simulation. Our main goal is to investigate the characteristics of the highest magnitude vibrational motion of wave front atoms at different heating pulses and ascertain their correspondence to a single longitudinal optical phonon. To this end it is shown that in the MD model, the equations of heat flux taken through the boundaries of a corresponding sampling area can resolve coherent phonon motion with high resolution when translational and vibrational modes are evaluated separately. Such a definition of heat flux allows the tracing of formation and dynamics of a single phonon. It is applied for different times of heating of a nanoribbon sample. The mechanism underlying the decay of phonons into diffusion is also probed, and energy conversion over the nanoribbon is evaluated. The relevant size of the area for the temporal and spatial flux resolution of a coherent phonon in the MD model is confirmed.

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

“…In the simplified MD model of a nanoribbon that we consider, without coupling to the electronic state, our system relates to the ground electronic state and non-resonant phonon generation. In our case, it is confirmed that the MD simulation alone is able to reproduce the coherent phonon generation and propagation in the anharmonic LJ potential of a nanoribbon with simultaneously present diffusive modes (13) .…”

“…The model was generalized by the nondimensional calculations using Lennard-Jones (LJ) potential. The Ar atoms are calculated, with potential parameters in the LJ potential being given (15) , details can be found in the Appendix (13) . The atoms in the calculation region were represented by Ar.…”

“…Their deviation defines the type of coherent phonons as the longitudinal optical one (13) . It should be mentioned that the propagating coherent phonon is not an equilibrium system.…”

Ballistic-diffusive phonon heat transport across grain boundaries

“…The nonequilibrium MD, or the direct method, mimics the heat source and sink in experiment, and then uses Fourier's law to quantify the thermal conductivity, nonetheless, only applies to diffusive heat conduction at steady-state [35][36][37]. In the past decades, a few attempts have been made to study the transient heat pulse propagation using MD thermostats to model the heat pulses [38][39][40][41]. However, thermostat algorithms thermalize phonons and tend to destroy phonon coherency [42], and hence are not suited for the simulation of the coherent phonon excitation by ultrafast laser pulse experiments.…”

A coherent phonon pulse model for transient phonon thermal transport

“…Unfortunately, neither method is well suited for the simulation of the generation and propagation of a phonon pulse. Existing attempts to study the transient heat pulse propagation have used the thermostat algorithms to model the heat source [33][34][35]. However, thermostat algorithms mimic thermalizing events such as damping and dephasing of phonon waves.…”