  We have shown theoretically that a combination of cross-section modulation
and acoustic mismatch in the core-shell Si/Ge nanowires can lead to a drastic
reduction of the thermal conductivity. Our calculations, which utilized two
different models - five-parameter Born-von Karman and six-parameter
valence-force field - for the lattice vibrations, indicate that the room
temperature thermal conductivity of Si/Ge cross-section modulated nanowires is
almost three orders of magnitude lower than that of bulk Si. Thermal flux in
the modulated nanowires is suppressed by an order of magnitude in comparison
with generic Si nanowires. The effect is explained by modification of the
phonon spectra in modulated nanowires leading to decrease of the phonon group
velocities and localization of certain phonon modes in narrow or wide nanowire
segments. The thermal conductivity inhibition is achieved in nanowires without
additional surface roughness and, thus, potentially reducing degradation of the
electron transport. Our results suggest that the acoustically mismatched
cross-section modulated nanowires are promising candidates for thermoelectric
applications.
