  Harnessing the power of low-dimensional materials in thermal applications
calls for a solid understanding of the anomalous thermal properties of such
systems. We analyze thermal conduction in one-dimensional systems by
determining the frequency-dependent phonon mean free paths (MFPs) for an
anharmonic chain, delivering insight into the diverging thermal conductivity
observed in computer simulations. In our approach, the MFPs are extracted from
the length-dependence of the spectral heat current obtained from nonequilibrium
molecular dynamics simulations. At low frequencies, the results reveal a
power-law dependence of the MFPs on frequency, in agreement with the diverging
conductivity and the recently determined equilibrium MFPs. At higher
frequencies, however, the nonequilibrium MFPs consistently exceed the
equilibrium MFPs, highlighting the differences between the two quantities.
Exerting pressure on the chain is shown to suppress the mean free paths and to
generate a weaker divergence of MFPs at low frequencies. The results deliver
important insight into anomalous thermal conduction in low-dimensional systems
and also reveal differences between the MFPs obtained from equilibrium and
nonequilibrium simulations.
