  Nano-engineering crystalline materials can be used to tailor their thermal
properties. By adding new nanoscale phonon scattering centers and controlling
their size, one can effectively decrease the phonon mean free path and hence
the thermal conductivity of a fully crystalline material. In this letter, we
use the 3$\omega$ method in the temperature range of 100-300 K to
experimentally report on the more than threefold reduction of the thermal
conductivity of an epitaxially-grown crystalline germanium thin film with
embedded polydispersed crystalline \ch{Ge3Mn5} nano-inclusions with diameters
ranging from 5 to 25~nm. A detailed analysis of the structure of the thin film
coupled with Monte Carlo simulations of phonon transport highlight the role of
the nano-inclusions volume fraction in the reduction of the phononic
contribution to the thermal conductivity, in particular its temperature
dependence, leading to a phonon mean free path that is set by geometrical
constraints.
