Micron-scale ballistic thermal conduction and suppressed thermal conductivity in heterogeneously interfaced nanowires

By employing three different measurement methods, we rigorously show that micron-scale ballistic thermal conduction can be found in Si-Ge heterogeneously interfaced nanowires exhibiting low thermal conductivities. The heterogeneous interfaces localize most high-frequency phonons and suppress the total thermal conductivity below that of Si or Ge. Remarkably, the suppressed thermal conductivity is accompanied with an elongation of phonon mean free paths over 5 \ensuremath{\mu}m at room temperature, which is not only more than 25 times longer than that of Si or Ge but also longer than those of the best thermal conductors like diamond or graphene. We estimate that only 0.1% of the excited phonons carry out the heat transfer process, and, unlike phonon transport in Si or Ge, the low-frequency phonons in Si-Ge core-shell nanowires are found to be insensitive to twin boundaries, defects, and local strain. The ballistic thermal conduction persisting over 5 \ensuremath{\mu}m, along with the suppressed thermal conductivity, will enable wave engineering of phonons at room temperature and inspire new improvements of thermoelectric devices.