# Soft phonons and ultralow lattice thermal conductivity in the Dirac semimetal Cd3As2

## TL;DR

This study reveals that soft optical phonon modes in Cd3As2, a Dirac semimetal, cause ultralow lattice thermal conductivity through enhanced phonon scattering, with temperature-dependent behavior linked to phonon interactions and potential Kohn anomalies.

## Contribution

It uncovers the role of soft phonon modes in low thermal conductivity of Cd3As2 using combined theoretical and experimental approaches, highlighting a novel mechanism involving Kohn anomalies.

## Key findings

- Soft optical phonon modes are present at the Brillouin zone center.
- These modes significantly increase phonon scattering, reducing thermal conductivity.
- The temperature dependence of thermal conductivity is explained by phonon-phonon interactions.

## Abstract

Recently, Cd3As2 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 Cd3As2 has an anomalously low lattice thermal conductivity, ranging from 0.3 W/mK 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 Cd3As2. 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 Cd3As2. Furthermore, we show that the interplay between the phonon-phonon Umklapp scattering rates and the soft optical phonon frequency explains the unusual non-monotonic temperature dependence of the lattice thermal conductivity of Cd3As2. 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.