Negative Magnetoresistance in Weyl semimetals NbAs and NbP: Intrinsic Chiral Anomaly and Extrinsic Effects

TL;DR

This review discusses how negative magnetoresistance in Weyl semimetals NbP and NbAs can originate from intrinsic chiral anomaly or extrinsic effects, emphasizing the importance of sample quality and complementary techniques for accurate interpretation.

Contribution

It clarifies the different origins of NMR in Weyl semimetals and highlights the limitations of using NMR alone to confirm Weyl fermions, advocating for combined approaches.

Findings

NMR can result from intrinsic chiral anomaly or extrinsic effects.

Sample quality heavily influences NMR and its temperature dependence.

Chemical doping provides a more reliable probe for Weyl fermions than NMR.

Abstract

Chiral anomaly induced negative magnetoresistance (NMR) has been widely used as a critical transport evidence on the existence of Weyl fermions in topological semimetals. In this mini review, we discuss the general observation of the NMR phenomena in non-centrosymmetric NbP and NbAs. We show that NMR can be contributed by intrinsic chiral anomaly of Weyl fermions and/or extrinsic effects, such as superimposition of Hall signals, field-dependent inhomogeneous current flow in the bulk, i.e. current jetting, and weak localization (WL) of coexistent trivial carriers. Such WL controlled NMR is heavily dependent on sample quality, and is characterized by pronounced crossover from positive to negative MR growth at elevated temperatures, as a result of the competition between the phase coherence time and the spin-orbital scattering constant of the bulk trivial pockets. Thus, the correlation of NMR and chiral anomaly needs to be scrutinized, without the support of other complimentary techniques. Due to the lifting of spin degeneracy, the spin orientations of Weyl fermions are either parallel or antiparallel to the momentum, a unique physical property known as helicity. The conservation of helicity provides strong protection for the transport of Weyl fermions, which can only be effectively scattered by magnetic impurities. Chemical doping of magnetic and non-magnetic impurities are thus more convincing in probing the existence of Weyl fermions than the NMR method.