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In ballistic transport, heat carriers such as phonons travel through thesolid without any scattering or interaction. Therefore, there is no temperaturegradient in the solid, which seems to transport the heat without getting heateditself. Ballistic transport is typically seen in high purity crystals at eithertemperatures below ~10 K, or physical size below ~100 nm, where the mean freepath of the carrier is larger than the solid itself. In this letter, we showevidence of ballistic transport at room temperature in lithium niobate wafersin the in-plane and cross-plane directions under both steady state and highfrequency heating that are monitored using both infrared and resistancethermometry. We report phonon mean free path in lithium niobate around 425microns, which is about 50 times higher than the largest phonon mean free pathin the literature at room temperature. Above this length-scale, temperaturegradient gradually emerges and the material shows completely diffusive, bulktransport at about 4 mm length. Our observations will impact phonon-basedelectronics such as thermal transistor, thermal logic gate and memory currentlyimpossible at room temperature. If 1 micron electron mean free path in graphenegives the highest-mobility, the 425 microns mean free path of phonons in thisresearch may realize phononics without any need for nanoscale size orultra-cold temperatures.

Evidence of Ballistic Thermal Transport in Lithium Niobate at Room Temperature