(37b) Heat Transfer in Quasi-One-Dimensional Nanostructures: Effects of Nonlinear Lattice Vibration Modes

The steady decrease of the feature size of integrated circuits leads to an increase in generated heat per unit area. Hence, efficient transfer of heat away from hotspots of integrated circuits becomes a crucial issue in the performance, stability, and design of new generations of electronic devices. One of the challenges in prediction of thermal properties of nanomaterials is that the classical Fourier law of thermal conductivity does not hold on this small scale. In this talk, we present results of theoretical modeling and molecular dynamic simulations of heat transport in quasi-one-dimensional nanostructures, focusing on carbon nanotubes. The study is performed under the assumption that the contribution of electrons to thermal conductivity is negligible and therefore the heat transfer is solely due to nonlinear interactions between vibrations of atoms in a nanostructure.

Recent research has shown that low-dimensional nanomaterials possess high thermal conductivity and hence are promising candidates for efficient heat reduction in nanodevices. Moreover, it is known that in certain model one-dimensional systems, such as the classical Fermi-Pasta-Ulam (FPU) model, in addition to linear lattice vibration modes (phonons), strongly non-linear modes (breathers) play an important role in the heat transport. These nonlinear spatially-localized modes have properties qualitatively different from the linear phonon vibration modes. In particular, they are stable and have long life time and lead to ballistic heat transfer responsible for anomalous thermal conductivity in FPU system. We have shown that the magnitude of nonlinear interactions within carbon nanotubes is comparable to that in the FPU model and therefore it is expected that strongly nonlinear modes play significant role in the thermal transport in the carbon nanotubes. We obtain and systematically classify these nonlinear modes by solving the steady-state equations of motion for the nanotube atoms. The stability of the obtained nonlinear modes is further analyzed by Floquet theory. Once the stable nonlinear modes are identified, we explore their role in the thermal transport by performing molecular dynamics simulations with initial conditions corresponding to selectively excited stable nonlinear modes. To further investigate the role of the nonlinear lattice vibration modes we perform non-equilibrium molecular dynamics simulations and study the evolution and interactions between the nonlinear modes.