Comparative study of phonon-limited carrier transport in the Weyl semimetal TaAs family

TL;DR

This work addresses why phonon-limited electrical transport differs among the TaAs family of Weyl semimetals. It employs an ab initio Boltzmann transport approach with electron-phonon coupling from EPW/Wannier interpolation to compute $\sigma_{xx}$ as a function of temperature and doping, relating it to $\sigma_{xx} \propto N(\varepsilon_{\rm F}) \langle v^2_x \rangle \tau(\varepsilon_{\rm F})$. The study finds NbP exhibits the highest conductivity due to large carrier velocities that offset stronger scattering, TaAs the lowest due to reduced pockets and velocities, and NbAs/NbP occupying intermediate regimes; electron-hole asymmetry is prominent in TaAs, whereas NbP shows doping robustness. Overall, the results provide a microscopic understanding of phonon-limited transport in Weyl semimetals and a framework applicable to other topological materials, highlighting the roles of phonons, doping, and carrier dynamics.

Abstract

We present a systematic first-principles study of phonon-limited transport in the TaAs family of Weyl semimetals using the ab initio Boltzmann transport equation. The calculated electrical conductivities show excellent agreement with experimental data for high-quality samples, confirming that transport in these systems is predominantly limited by phonon scattering. Among the four compounds, NbP achieves the highest conductivity, governed primarily by its large Fermi velocities that offset its stronger scattering rates. In contrast, TaAs displays the lowest conductivity, linked to reduced carrier pockets and limited carrier velocities. Additionally, NbP conductivity remains largely unaffected by small hole or electron doping, whereas TaAs exhibits pronounced electron-hole asymmetry. NbAs and TaP show intermediate behavior, reflecting their Fermi surface topologies and scattering phase space. These findings provide microscopic insight into the transport mechanisms of the TaAs family and emphasize the critical role of phonons, doping, and carrier dynamics in shaping their electronic response.

Figures (6)

• Figure 1: Electronic band structures with orbital-resolved character and corresponding total and partial density of states (DOS) for (a) TaAs, (b) TaP, (c) NbAs, and (d) NbP. Contributions from Ta/Nb and As/P orbitals are shown in blue and red colors, respectively, while the total DOS is represented by the gray shaded area.
• Figure 2: Fermi surfaces within a 10 meV energy window around the Fermi level for (a) TaAs, (b) TaP, (c) NbAs, and (d) NbP, generated using XCrySDen Kokalj1999. Yellow and magenta regions indicate hole and electron pockets. The black solid arrows in panel (a) indicate possible inter-pocket scatterings with $\mathbf{q}_1$ and $\mathbf{q}_2$ wave vectors along the $\Gamma$-$Z$ and $\Gamma$-$X$ directions (dashed lines). (e) Nesting functions along the same high-symmetry path for all four compounds.
• Figure 3: Phonon band structures and phonon density of states (PhDOS) for (a) TaAs, (b) TaP, (c) NbAs, and (d) NbP. Contributions from Ta/Nb and As/P orbitals are shown in blue and red, respectively, while the total PhDOS is represented by the gray shaded area.
• Figure 4: Electrical conductivity $\sigma_{xx}$ as a function of temperature for (a) TaAs, (b) TaP, (c) NbAs, and (d) NbP for three different Fermi level positions: 0 meV (black triangles), $-$25 meV (blue circles), and +25 meV (red circles). Solid symbols denote our calculated results, while hollow symbols represent the experimental data from Refs. Zhang2017Huang2015Xiang2017Zhang2015Arnold2016Ghimire2015Shekhar2015Wang2016Sankar2018Balduini2024. In panels (a), (b), and (d), multiple hollow markers represent different samples reported in Ref. Zhang2017 for TaAs, and Refs. Zhang2015Wang2016 for TaP and NbP. The distinct hollow symbols illustrate how the sample quality affects the transport properties.
• Figure 5: (a) Electrical conductivity $\sigma_{xx}$ from SERTA (dashed lines, open squares) and IBTE (solid lines, filled circles) for TaAs, TaP, NbAs, and NbP in the undoped case. (b) Density of states (DOS), (c) energy-averaged scattering rates at 300 K, and (d) average squared Fermi velocity $\langle v_x^2\rangle$ for all four compounds.