Negative magnetoresistance in Weyl semimetals NbAs and NbP: Intrinsic chiral anomaly and extrinsic effects

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.

Universal scattering response across the type-II Weyl semimetal phase diagram

“…It is long known that disorder, and especially resonant impurities, can significantly impact onto transport properties. Even recently, the presence of defects has been invoked to be at the origin of both positive as well as negative magnetoresistance effects in topological semimetals [51,52]. In this context, our observations contribute by providing a detailed microscopic picture of the resonant scattering off impurities in type-II Weyl semimetals.…”

Bipolar Conduction as the Possible Origin of the Electronic Transition in Pentatellurides: Metallic vs Semiconducting Behavior

“…As can be seen, the negative MR becomes prominent for T ≥ 10 K at μ 0 H > 1 T. The results are similar to those reported by Li et al [12], and they were interpreted in terms of the chiral magnetic effect. However, as far as we know, negative MR might also come from several other extrinsic effects, such as the superimposition of Hall signals, fielddependent inhomogeneous current flow in the bulk, and weak localization of coexistent trivial carriers [28,29].…”

Uncovering electron-phonon scattering and phonon dynamics in type-I Weyl semimetals

“…2,[5][6][7][8][9][10][11][12][13][14][15][16] From this discovery sprang reports of the linear and nonlinear optical response of these materials [17][18][19][20][21][22][23] as well as many magnetotransport studies seeking definitive signatures of the chiral anomaly [24][25][26][27] in this family [28][29][30][31][32][33][34][35][36] for further confirmation of the Weyl semimetal phase. While the observation of the chiral anomaly is still a subject of active debate, 2,37,38 reports of these materials' giant magnetoresistance and ultrahigh mobilities make their transport a topic of fundamental importance. [14][15][16]28,[30][31][32][33][34][35][36]39…”