Landau diamagnetism and Weyl-fermion excitations in TaAs revealed by As 75 NMR and NQR

The electronic and superconducting properties associated with the topologically nontrivial bands in Weyl semimetals have recently attracted much attention. We report the microscopic properties of the type-I Weyl semimetal TaAs measured by As75 nuclear magnetic (quadrupole) resonance under zero and elevated magnetic fields over a wide temperature range up to 500 K. The magnetic susceptibility measured by the Knight shift K is found to be negative at low magnetic fields and has a strong field (B) dependence as lnB at T=1.56K. Such nonlinear field-dependent magnetization can be well accounted for by Landau diamagnetism arising from the three-dimensional linearly dispersed bands, and thus is a fingerprint of topological semimetals. We further study the low-energy excitations by the spin-lattice relaxation rate 1/T1. At zero field and 30≤T≤250K, 1/T1T shows a T2 variation due to Weyl-node excitations. At B∼13T, 1/T1T exhibits the same T dependence but with a smaller value, scaling with K2 T2, which indicates that the Korringa relation also holds for a Weyl semimetal. Analysis of the Korringa ratio reveals that the energy range of the linear bands is about 250 K in TaAs.