Spectroscopic and first-principles investigation of TaAs Weyl semimetal structure and optical characteristics

We present a synthesis-characterization-theory benchmark for the Weyl semimetal tantalum arsenide (TaAs) that targets the microscopic origin of its anisotropic optical response. Using an optimized iodine assisted chemical vapor transport (CVT) protocol (1020/920 °C gradient, 7-day transport), we reproducibly obtain phase-pure, faceted TaAs single crystals with dimensions up to 7 mm. Single-crystal X-ray diffraction (SC-XRD), Raman spectroscopy, and energy-dispersive X-ray spectroscopy (EDX) confirm the expected non-centrosymmetric I4₁md (No. 109) structure and near-stoichiometric composition. On the theory side, we compute the electronic structure and anisotropic dielectric function using two independent first-principles formalisms: frequency-domain DFT linear response (RPA, epsilon.x) and real-time TDDFT (turboTDDFT). The consistency between these approaches provides a stringent validation of the calculated dielectric tensor and enables an orbital-resolved assignment of the strongest optical resonances. We find that the dominant inter-band features in the 2-6 eV range arise primarily from hybridized Ta 5d-As 4p transitions, whereas direct contributions from Weyl-node states are comparatively weak in the linear optical spectra. These results establish that the dominant optical and resonant nonlinear responses of TaAs are governed primarily by anisotropic Ta 5d-As 4p inter-band transitions rather than by Weyl-node states themselves, providing a clear microscopic framework for interpreting its giant second harmonic generation (SHG) and guiding future optoelectronic applications of polar Weyl semimetals.