Exotic Landau Diamagnetism and Weyl-Fermions Excitations in TaAs Revealed by $^{75}$As NMR and NQR

TL;DR

This study uses $^{75}$As NMR and NQR to investigate the microscopic electronic properties of the Weyl semimetal TaAs, revealing Landau diamagnetism and Weyl fermion excitations through temperature and magnetic field dependence.

Contribution

It provides the first microscopic NMR/NQR evidence of Landau diamagnetism and Weyl fermion excitations in TaAs, linking topological band features with measurable magnetic responses.

Findings

Landau diamagnetism observed via nonlinear field-dependent magnetization.

Weyl nodes excitations cause a T^2 variation in spin-lattice relaxation rate.

Korringa relation holds, indicating linear band energy range of about 250 K.

Abstract

The electronic and superconducting properties associated with the topologically non-trivial bands in Weyl semimetals have recently attracted much attention. We report the microscopic properties of the type-I Weyl semimetal TaAs measured by $^{75}$As 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 have a strong field ($B$) dependence as ln$B$ at $T$ = 1.56 K. Such nonlinear field-dependent magnetization can be well accounted for by Landau diamagnetism arising from the 3D 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 K $\leq T\leq$ 250 K, 1/$T_{1}T$ shows a $T^{2}$ variation due to Weyl nodes excitations. At $B \sim$ 13 T, $1/T_1T$ exhibits the same $T$-dependence but with a smaller value, scaling with $K^2\propto T^2$, 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.