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 As-75 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 In B at T = 1.56 K. 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/T-1. At zero field and 30 <= T <= 250 K, 1/T1T shows a T-2 variation due to Weyl-node excitations. At B similar to 13 T, 1/T1T exhibits the same T dependence but with a smaller value, scaling with K-2 alpha T-2, 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.