Emergent Weyl fermion excitations in TaP explored by 181-Ta quadrupole resonance

Abstract: 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.