Optical Distinctions Between Weyl Semimetal TaAs and Dirac Semimetal Na3Bi: An Ab Initio Investigation

We present ab initio a study on linear and nonlinear optical properties of topological semimetal Tantalum arsenide and Sodium bismuthate. The real and imaginary part of the dielectric function in addition to the energy loss spectra of TaAs and Na 3 Bi have been calculated within random phase approximation (RPA); then, the electron–hole interaction is included by solving the Bethe–Salpeter equation for the electron–hole Green’s function. In spite of being in the single category of topological materials, we have found obvious distinction between linear optical responses of TaAs and Na 3 Bi at a high energy region where, in contrast to Na 3 Bi, Tantalum arsenide has excitonic peaks at 9 eV and 9.5 eV. It is remarkable that the excitonic effects in the high energy range of the spectrum are stronger than in the lower one. The dielectric function is overall red shifted compared with that of RPA approximation. The resulting static dielectric constants for Na 3 Bi are smaller than corresponding ones in TaAs. At a low energy region, the absorption intensity of TaAs is more than Na 3 Bi. The calculated second-order nonlinear optical susceptibilities χ ijk (2) ( ω ) show that Tantalum arsenide acts as a Weyl semimetal, and has high values of nonlinear responses in the low energy region which makes it promising candidate for the second harmonic generation in the terahertz frequency region. In the low energy regime, optical spectra are dominated by the 2 ω intra-band contributions.