Emergent Weyl fermion excitations in TaP explored by 181-Ta quadrupole   resonance
  The $^{181}$Ta quadrupole resonance (NQR) technique has been utilized to investigate the microscopic magnetic properties of the Weyl semi-metal TaP. We found three zero-field NQR signals associated with the transition between the quadrupole split levels for Ta with $I$=7/2 nuclear spin. A quadrupole coupling constant, $\nu_{\mathrm{Q}}$ =19.250 MHz, and an asymmetric parameter of the electric field gradient, $\eta$ = 0.423 were extracted, in good agreement with band structure calculations. In order to examine the magnetic excitations, the temperature dependence of the spin lattice relaxation rate (1/$T_{\mathrm{1}}T$) has been measured for the $f_{\mathrm{2}}$-line ($\pm$5/2 $\leftrightarrow$ $\pm$3/2 transition). We found that there exists two regimes with quite different relaxation processes. Above $T\text{*}$ $\approx$ 30 K, a pronounced (1/$T_{\mathrm{1}}T$) $\propto$ $T^{2}$ behavior was found, which is attributed to the magnetic excitations at the Weyl nodes with temperature dependent orbital hyperfine coupling. Below $T\text{*}$, the relaxation is mainly governed by Korringa process with 1/$T_{\mathrm{1}}T$ = constant, accompanied by an additional $T^{-1/2}$ type dependence to fit our experimental data. We show that Ta-NQR is a novel probe for the bulk Weyl fermions and their excitations. 
H. Yasuoka, T. Kubo, Y. Kishimoto, D. Kasinathan, M. Schmidt, B. Yan,   Y. Zhang, H. Tou, C. Felser, A. P. Mackenzie, M. Baenitz
cond-mat.mtrl-sci cond-mat.str-el