Emergent Weyl Fermion Excitations in TaP Explored by Ta 181 Quadrupole Resonance

The Ta181 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, νQ=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) 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.