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Based on our first detailed $^{181}$Ta nuclear quadrupole resonance (NQR)studies from 2017 on the Weyl semimetal TaP, we now extended our NQR studies toanother Ta-based monopnictide TaAs. In the present work, we have determined thetemperature-dependent $^{181}$Ta-NQR spectra, the spin-lattice relaxation time$T_{1}$, and the spin-spin relaxation time $T_{2}$. We found the followingcharacteristic features that showed great contrast to what was found in TaP:(1) The quadrupole coupling constant and asymmetry parameter of EFG, extractedfrom three NQR frequencies, have a strong temperature dependence above $\sim$80K that cannot be explained by the density functional theory calculationincorporating the thermal expansion of the lattice. (2) The temperaturedependence of the spin-lattice relaxation rate, $1/T_{1} T$, shows a $T^{4}$power law behavior above $\sim$30 K. This is a great contrast with the $1/T_{1}T \propto T^{2}$ behavior found in TaP, which was ascribed to the magneticexcitations at the Weyl nodes with a temperature-dependent orbital hyperfinecoupling. (3) Regarding the nuclear spin-spin interaction, we found thespin-echo signal decays with the pulse separation simply by a Lorentzianfunction in TaAs, but we have observed spin-echo modulations in TaP that ismost likely due to the indirect nuclear spin-spin coupling via virtuallyexcited Weyl fermions. From our experimental findings, we conclude that thepresent NQR results do not show dominant contributions from Weyl fermionexcitations in TaAs.

Comparative $^{181}$Ta-NQR Study of Weyl Monopnictides TaAs and TaP: Relevance of Weyl Fermion Excitations