Topological phonons and thermal conductivity of two-dimensional Dirac semimetal PtN4C2.

PtN₄C₂ is a recently predicted two-dimensional (2D) Dirac semimetal exhibiting significant topological quantum spin and valley Hall effects. Herein, we explore its topological phonon states and thermal transport properties from first-principles calculations. In terms of symmetry arguments, we predict the existence of multiple topologically protected phononic Dirac points in the frequency range of 0–20 THz, which are evidenced by the relevant irreducible representations and calculated nontrivial edge states on the (100) surface. In addition, anharmonic phonon renormalization is found to play a significant role in determining the phonon spectrum, especially for the out-of-plane flexural acoustic (ZA) branch. Moreover, we explicitly consider three-phonon scattering, four-phonon scattering, and phonon renormalization to predict the lattice thermal conductivity κ_l of PtN₄C₂, by solving the Boltzmann transport equation. With the incorporation of four-phonon scattering, we predict that the intrinsic κ_l is 68 W/mK at room temperature, which is reduced by about 45% as compared to the value obtained by only including three-phonon scattering. This reduction is found to arise mainly from the ZA phonons, whose contribution to κ_l is significantly suppressed by four-phonon scattering, due to the restriction of the mirror symmetry-induced selection rules on three-phonon processes. We also unveil that the presence of Dirac points steepens the surrounding phonon dispersion and thus greatly increases the phonon group velocities, thereby making a considerable contribution to κ_l. This work establishes a thorough understanding of intrinsic topological phonons and thermal transport in PtN₄C₂ and highlights the importance of phonon renormalization and higher-order anharmonicity in determining the phonon transport properties of 2D materials. [ABSTRACT FROM AUTHOR]