Nondiffusive lattice thermal transport in Si-Ge alloy nanowires
We present a calculation of the lattice thermal conductivity of Si-Ge nanowires (NWs), based on solving the Boltzmann transport equation by the Monte Carlo method of sampling the phonon mean free paths. We augment the previous work with the full phonon dispersion and a partially diffuse momentum-dependent specularity model for boundary roughness scattering. We find that phonon flights are comprised of a mix of long free flights over several $\ensuremath{\mu}\mathrm{m}$ interrupted by bursts of short flights, resulting in a heavy-tailed distribution of flight lengths, typically encountered in L\'evy walk dynamics. Consequently, phonon transport in Si-Ge NWs is neither entirely ballistic nor diffusive; instead, it falls into an intermediate regime called superdiffusion where thermal conductivity scales with the length of the NW as $\ensuremath{\kappa}\ensuremath{\propto}{L}^{\ensuremath{\alpha}}$ with the exponent of length dependence $\ensuremath{\alpha}\ensuremath{\approx}0.33$ over a broad range of wire lengths $10\phantom{\rule{4pt}{0ex}}\mathrm{nm}lLl10\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{m}$ regardless of diameter and roughness. We conclude that thermal conductivity in Si-Ge alloy NWs is length dependent up to $10 \ensuremath{\mu}\mathrm{m}$ and therefore can be tuned for thermoelectric applications.