Soft phonons and ultralow lattice thermal conductivity in the Dirac semimetal Cd_{3}As_{2}

Recently, Cd_{3}As_{2} has attracted intensive research interest as an archetypical Dirac semimetal, hosting three-dimensional linear-dispersive electronic bands near the Fermi level. Previous studies have shown that single-crystalline Cd_{3}As_{2} has an anomalously low lattice thermal conductivity, ranging from 0.3 to 0.7 W/mK at 300 K, which has been attributed to point defects. In this work, we combine first-principles lattice dynamics calculations and temperature-dependent high-resolution Raman spectroscopy of high-quality single-crystal thin films grown by molecular-beam epitaxy to reveal the existence of a group of soft optical phonon modes at the Brillouin-zone center of Cd_{3}As_{2}. These soft phonon modes significantly increase the scattering phase space of heat-carrying acoustic phonons and are the origin of the low lattice thermal conductivity of Cd_{3}As_{2}. Furthermore, we show that the interplay between the phonon-phonon Umklapp scattering rates and the soft optical phonon frequency explains the unusual nonmonotonic temperature dependence of the lattice thermal conductivity of Cd_{3}As_{2}. Our results further suggest that the soft phonon modes are potentially induced by a Kohn anomaly associated with the Dirac nodes, in analogy to similar, nonetheless weaker, effects in graphene and Weyl semimetals.