  From nano-scale heat transfer point of view, currently one of the most
interesting and challenging tasks is to quantitatively analyzing phonon mode
specific transport properties in solid materials, which plays vital role in
many emerging and diverse technological applications. It has not been so long
since such information can be provided by the phonon spectral energy density
(SED) or equivalently time domain normal mode analysis (TDNMA) methods in the
framework of equilibrium molecular dynamics simulation (EMD). However, until
now it has not been realized in non-equilibrium molecular dynamics simulations
(NEMD), the other widely used computational method for calculating thermal
transport of materials in addition to EMD. In this work, a computational scheme
based on time Fourier transform of atomistic heat current, called frequency
domain direct decomposed method (FDDDM), is proposed to analyze the
contributions of frequency dependent thermal conductivity in NEMD simulations.
The FDDDM results of Lennard-Jones (LJ) Argon and Stillinger-Weber (SW) Si are
compared with TDNMA method from EMD simulation. Similar trends are found for
both cases, which confirm the validity of our FDDDM approach. Benefiting from
the inherent nature of NEMD and the theoretical formula that does not require
any temperature assumption, the FDDDM can be directly used to investigate the
size and temperature effect. Moreover, the unique advantage of FDDDM prior to
previous methods (such as SED and TDNMA) is that it can be straightforwardly
used to characterize the phonon frequency dependent thermal conductivity of
disordered systems, such as amorphous materials. The FDDDM approach can also be
a good candidate to be used to understand the phonon behaviors and thus
provides useful guidance for designing efficient structures for advanced
thermal management.
