Anomalously High Phonon Thermal Conductivity Driven by Weak Electron-Phonon Coupling in Weyl Semimetals TaAs and TaP

TL;DR

The paper investigates thermal transport in TaAs and TaP and finds phonon-dominated heat conduction, with TaP reaching κ_ph ≈ 171 W/mK along the a-axis at room temperature and exceeding κ_e by more than a factor of five. It performs first-principles calculations of both phonon and electron transport by solving the phonon Boltzmann transport equation (via ShengBTE) and the electron Boltzmann transport equation (via Perturbo), using electron-phonon matrix elements from EPW. A low electronic density of states near the Fermi level arising from Weyl topology weakens phonon–electron coupling and, together with acoustic phonon bunching and a wide acoustic–optical gap, suppresses scattering, boosting κ_ph and causing the Lorenz number to deviate from the Sommerfeld value $L_0 = \frac{\pi^{2} k_B^{2}}{3 e^{2}}$. The findings suggest phonon-mediated transport is a universal feature of topological semimetals and could guide discovery of materials with high thermal conductivity.

Abstract

In conventional metals, thermal transport is governed by electrons, with phonon contributions often considered negligible. Here, through rigorous first-principles calculations, we uncover a phonon-dominated thermal transport regime in the Weyl semimetals TaAs and TaP. Remarkably, although TaP is metallic, its phonon thermal conductivity ($κ_{\text{ph}}$) reaches as high as 171 Wm$^{-1}$K$^{-1}$ at room temperature, surpassing its electronic counterpart by more than a factor of five. This anomalously high $κ_{\text{ph}}$ is enabled by the unique electronic and phononic band structures, characterized by the Weyl nodes near the Fermi level, together with acoustic phonon bunching and a wide frequency gap in the phonon spectrum, which collectively suppress phonon-electron and phonon-phonon scattering processes. Due to the substantial phonon contribution, the derived Lorenz number deviates strongly from the conventional Wiedemann-Franz law. We further show that the significance of phonon thermal transport is universal across topological semimetals. Our work provides deeper insight into thermal transport mechanisms in topological semimetals and extends the scope for discovering materials with high thermal conductivity.

Figures (5)

• Figure 1: (a) The primitive cell and Brillouin Zone of TaX (X = As, P). Phonon dispersion relation and projected phonon density of states (DOS) of (b) TaAs and (d) TaP, where the phonon linewidth due to ph-el scattering is projected onto the corresponding phonon structure. Electronic band structure and electronic DOS of (c) TaAs and (e) TaP without and with including spin-orbital coupling (SOC).
• Figure 2: Temperature-dependent $\kappa_{\rm{ph}}$ of (a) TaAs and (b) TaP with different combinations of scattering mechanisms. The filled and empty symbols correspond to the $\kappa_{\rm ph}$ along the $a$ and $c$ axes, respectively.
• Figure 3: Calculated spectral contributions to the $\kappa_{\rm ph}$ of (a) TaAs and (c) TaP along the a-axis at RT. The phonon scattering rates contributed from 3ph, 4ph, ph-iso, and ph-el processes for (b) TaAs and (d) TaP at RT.
• Figure 4: (a, d) The electronic conductivity, (b, e) the $\kappa_{e}$, $\kappa_{\mathrm{ph}}$, and total thermal conductivity ($\kappa = \kappa_{\mathrm{ph}} + \kappa_{e}$), and (c, f) the Lorenz number as a function of temperature for TaAs and TaP, respectively.
• Figure 5: The relative importance of $\kappa_{\rm ph}$, as gauged by the ratio $\kappa_{\rm ph}$/$\kappa_{\rm e}$, for available topological semimetals and metals; the colorbar represents the corresponding el-ph coupling strength ($\lambda$). The presented data for the electronic DOS, $\kappa_{\rm ph}$, $\kappa_{\rm e}$, and $\lambda$ are extracted from Refs. PhysRevB.100.144306kundu2021ultrahighPhysRevLett.123.136802 and the Material Project database 10.1063/1.4812323munro2020improved. Topological semimetals are marked with stars.