Comprehensive anisotropic linear optical properties of the Weyl semimetals TaAs and NbAs
Rui Zu, Mingqiang Gu, Lujin Min, Chaowei Hu, Ni Ni, Zhiqiang Mao, James M. Rondinelli, Venkatraman Gopalan
Abstract
TaAs and NbAs are two of the earliest identified Weyl semimetals that possess many intriguing optical properties, such as chirality-dependent optical excitations and giant second harmonic generation (SHG). Linear and nonlinear optics have been employed as tools to probe the Weyl physics in these crystals. Here, we extend these studies to address two important aspects: determining the complete anisotropic dielectric response and exploring if and how they can reveal essential Weyl physics. We determine the complete anisotropic dielectric functions of TaAs and NbAs by combining spectroscopic ellipsometry and density functional theory (DFT). Parameterized Lorentz oscillators are reported from 1.2--6 eV (experiment) and 0--6 eV (DFT), and good agreement is shown between them. Both linear and nonlinear optical properties have been previously reported to reveal Weyl physics. We suggest that strong optical resonances from trivial bands are the likely origin of the large optical SHG previously reported at these energies. Furthermore, by comparing the contribution of a small $k$-space centered around the Weyl cones with the total linear dielectric function, we find that these contributions are highly anisotropic and are $<25%$ of the total dielectric function $\mathrm{below}\phantom{\rule{0.16em}{0ex}}0.5\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$; $\mathrm{above}\phantom{\rule{0.16em}{0ex}}1\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$, these contributions are negligible. Thus, the study of Weyl physics using optical techniques requires very low energies, and even there, a careful assessment is required in distinguishing the much smaller contributions of the Weyl bands from the dominant contributions of the trivial bands and Drude response to the total dielectric function.