Phonon second sound over 200 K and cubic boron arsenide as a superior semiconductor

This talk will report our observations of phonon second sound over 200K in graphite, and the simultaneously high phonon thermal conductivity, and electron and hole mobilities in cubic boron arsenide. The experimental studies were stimulated by first-principles based simulations on electron and phonon transport. Direct solutions of the phonon Boltzmann transport equation, including the full scattering matrix obtained from first-principles simulations of phonon-phonon scattering, predicted the existence of phonon hydrodynamic transport at high temperatures. Our experiments using transient grating setup observed second sound, the collective thermal transport of phonons as waves, over 200 K. Our experiments and simulations also show the co-existence of thermal zero sound—the thermal waves due to ballistic phonons. The talk will then shift to phonon and electron transport in cubic boron arsenide. First-principles simulations, including 3- and 4-phonon scattering processes, and electron-phonon interaction, predicted that the special phonon band structure of cubic boron arsenide lead to very high phonon thermal conductivities, and simultaneously high electron and hole mobilities. These predictions were confirmed by experiments, with measured room-temperature thermal conductivity over 1200 W/m-K, and ambipolar mobility over 1600 cm²/s. With a bandgap ~2 eV, these properties position the cubic boron arsenide as the best semiconductor.