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

The ¹⁸¹Ta quadrupole resonance [nuclear quadrupole resonance (NQR)] technique is utilized to investigate the microscopic magnetic properties of the Weyl semimetal TaP. We find 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, ν_Q=19.250 MHz, and an asymmetric parameter of the electric field gradient, η=0.423, are 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/T₁T) is measured for the f₂ line (±5/2↔±3/2 transition). We find that there exist two regimes with quite different relaxation processes. Above T^∗≈30 K, a pronounced (1/T₁T) α T² behavior is found, which is attributed to the magnetic excitations at the Weyl nodes with temperature-dependent orbital hyperfine coupling. Below T^∗, the relaxation is mainly governed by a Korringa process with 1/T₁T=const, 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.