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

TL;DR

This study compares the nuclear quadrupole resonance properties of Weyl semimetals TaAs and TaP, revealing contrasting temperature-dependent behaviors and suggesting limited influence of Weyl fermion excitations in TaAs.

Contribution

First detailed NQR comparison of TaAs and TaP, highlighting differences in electronic and magnetic properties related to Weyl fermions.

Findings

Temperature dependence of EFG parameters in TaAs differs from TaP.

Spin-lattice relaxation rate in TaAs follows a T^4 law, unlike T^2 in TaP.

No dominant Weyl fermion contributions observed in TaAs NQR data.

Abstract

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 to another Ta-based monopnictide TaAs. In the present work, we have determined the temperature-dependent $^{181}$Ta-NQR spectra, the spin-lattice relaxation time $T_{1}$, and the spin-spin relaxation time $T_{2}$. We found the following characteristic features that showed great contrast to what was found in TaP: (1) The quadrupole coupling constant and asymmetry parameter of EFG, extracted from three NQR frequencies, have a strong temperature dependence above $\sim$80 K that cannot be explained by the density functional theory calculation incorporating the thermal expansion of the lattice. (2) The temperature dependence 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 magnetic excitations at the Weyl nodes with a temperature-dependent orbital hyperfine coupling. (3) Regarding the nuclear spin-spin interaction, we found the spin-echo signal decays with the pulse separation simply by a Lorentzian function in TaAs, but we have observed spin-echo modulations in TaP that is most likely due to the indirect nuclear spin-spin coupling via virtually excited Weyl fermions. From our experimental findings, we conclude that the present NQR results do not show dominant contributions from Weyl fermion excitations in TaAs.