  Thermal transport in crystals is influenced by chemistry, boundaries, and
nanostructure. The phonon band structure extracted from molecular-dynamics
simulations provides an illuminating view of both the type and extent of
prevalence of wavelike mechanisms underlying the transport, yet falls short of
elucidating the nature of thermal evolution for different phonon regimes. Here
we present an analysis framework for the characterization of the entropic
signature of the mechanisms induced by the existence of boundaries and
nanostructure, using both equilibrium and nonequilibrium atomistic simulations.
Specifically, we examine the effects of phonon confinement, Bragg scattering,
and local resonances on the configurational phase space in room-temperature
nanostructured silicon, and quantify how each modifies the rate of entropy
production and thermal relaxation. We reveal that the presence of phonon local
resonances spanning the full spectrum enables a highly ordered regime of heat
conduction, where irreversible evolution and entropy maximization are severely
hindered by extensive mode hybridizations caused by the resonances. This unique
regime of transport paves the way for ultra-precise phonon control for a wide
range of applications in condensed matter physics.
