  In ballistic transport, heat carriers such as phonons travel through the
solid without any scattering or interaction. Therefore, there is no temperature
gradient in the solid, which seems to transport the heat without getting heated
itself. Ballistic transport is typically seen in high purity crystals at either
temperatures below ~10 K, or physical size below ~100 nm, where the mean free
path of the carrier is larger than the solid itself. In this letter, we show
evidence of ballistic transport at room temperature in lithium niobate wafers
in the in-plane and cross-plane directions under both steady state and high
frequency heating that are monitored using both infrared and resistance
thermometry. We report phonon mean free path in lithium niobate around 425
microns, which is about 50 times higher than the largest phonon mean free path
in the literature at room temperature. Above this length-scale, temperature
gradient gradually emerges and the material shows completely diffusive, bulk
transport at about 4 mm length. Our observations will impact phonon-based
electronics such as thermal transistor, thermal logic gate and memory currently
impossible at room temperature. If 1 micron electron mean free path in graphene
gives the highest-mobility, the 425 microns mean free path of phonons in this
research may realize phononics without any need for nanoscale size or
ultra-cold temperatures.
