  Pure amorphous solids are traditionally considered to set the lower bound of
thermal conductivity due to their disordered atomic structure that impedes
vibrational energy transport. However, the lower limits for thermal
conductivity in heterogeneous amorphous solids and the physical mechanisms
underlying these limits remain unclear. Here, we use equilibrium molecular
dynamics to show that an amorphous SiGe nanocomposite can possess thermal
conductivity substantially lower than those of the amorphous Si and Ge
constituents. Normal mode analysis indicates that the presence of the Ge
inclusion localizes vibrational modes with frequency above the Ge cutoff in the
Si host, drastically reducing their ability to transport heat. This observation
suggests a general route to achieve exceptionally low thermal conductivity in
fully dense solids by restricting the vibrational density of states available
for transport in heterogeneous amorphous nanocomposites.
