Nuclear spin-lattice relaxation time in TaP and the Knight shift of Weyl semimetals

TL;DR

This paper analyzes nuclear spin-lattice relaxation and Knight shift in Weyl semimetals, explaining temperature dependence and doping effects, and predicts unique signatures for identifying Weyl points.

Contribution

It develops a theoretical framework for the Knight shift in Weyl semimetals, including temperature and doping effects, and connects these to experimental relaxation data.

Findings

Non-monotonic temperature dependence explained by chemical potential shifts

Knight shift can vanish or change sign with doping or temperature

Unusual energy dependence of the Korringa relation in Weyl semimetals

Abstract

We first analyze the recent experimental data on the nuclear spin-lattice relaxation rate of the Weyl semimetal TaP. We argue that its non-monotonic temperature dependence is explained by the temperature dependent chemical potential of Weyl fermions. We also develop the theory of the Knight shift in Weyl semimetals, which contains two counteracting terms. The diamagnetic term follows $-\ln[W/\max(|\mu|,k_BT)]$ with $W$, $\mu$ and $T$ being the high energy cutoff, chemical potential and temperature, respectively, and is always negative. The paramagnetic term scales with $\mu$ and changes sign depending on the doping level. Altogether, the Knight shift is predicted to vanish or even change sign upon changing the doping or the temperature, making it a sensitive tool to identify Weyl points. We also calculate the Korringa relation for Weyl semimetals which shows an unusual energy dependence rather than being constant as expected for a non-interacting Fermi system.