Negative magnetoresistance without well-defined chirality in the Weyl semimetal TaP (1506.06577v2)

Abstract: Weyl semimetals (WSMs) are topological quantum states wherein the electronic bands linearly disperse around pairs of nodes, the Weyl points, of fixed (left or right) chirality. The recent discovery of WSM materials triggered an experimental search for the exotic quantum phenomenon known as the chiral anomaly. Via the chiral anomaly nonorthogonal electric and magnetic fields induce a chiral density imbalance that results in an unconventional negative longitudinal magnetoresistance, the chiral magnetic effect. Recent theoretical work suggests that this effect does not require well-defined Weyl nodes. Experimentally however, it remains an open question to what extent it survives when chirality is not well-defined, for example when the Fermi energy is far away from the Weyl points. Here, we establish the detailed Fermi surface topology of the recently identified WSM TaP via a combination of angle-resolved quantum oscillation spectra and band structure calculations. The Fermi surface forms spin-polarized banana-shaped electron and hole pockets attached to pairs of Weyl points. Although the chiral anomaly is therefore ill-defined, we observe a large negative magnetoresistance (NMR) appearing for collinear magnetic and electric fields as observed in other WSMs. In addition, we show experimental signatures indicating that such longitudinal magnetoresistance measurements can be affected by an inhomogeneous current distribution inside the sample in a magnetic field. Our results provide a clear framework how to detect the chiral magnetic effect.

• The paper reveals that negative magnetoresistance in TaP appears even without a distinct chiral anomaly, questioning conventional interpretations.
• The study employs quantum oscillation measurements and ab initio calculations to map TaP's Fermi surface, highlighting spin-polarized, banana-shaped electron and hole pockets.
• The findings suggest that extrinsic factors, such as inhomogeneous current distribution, may contribute to the observed negative magnetoresistance, urging further research.

An Analysis of Negative Magnetoresistance in the Weyl Semimetal TaP

In the paper "Negative magnetoresistance without well-defined chirality in the Weyl semimetal TaP," the authors explore the electronic properties of TaP, a newly identified Weyl semimetal (WSM). The paper examines the negative magnetoresistance (MR) characteristic in TaP, particularly when the concept of chirality is not well-defined due to the Fermi energy's position relative to Weyl points. This research seeks to address the experimental evidence of the chiral anomaly in Weyl semimetals.

Key Findings

The authors determine the Fermi surface topology of the TaP WSM using quantum oscillation experiments and ab initio band structure calculations. They publicize that the Fermi surface consists of spin-polarized banana-shaped electron and hole pockets that incorporate Weyl points, yet the chiral anomaly is not distinctly defined. Despite the absence of well-defined Weyl nodes typically necessary for the chiral anomaly, a large negative MR was consistently measured when magnetic and electric fields were collinear.

Furthermore, it is discussed that the apparent large negative MR in TaP can possibly be explained by contributions from other sources, such as an inhomogeneous current distribution inside the sample when subjected to a magnetic field. The researchers emphasize that these findings challenge the straightforward association of observed negative MR with the chiral anomaly in systems where the Fermi surface topology does not accommodate independent pockets around Weyl points.

Implications and Speculation

The results of this paper suggest that the negative MR observed in TaP and potentially other WSM materials might not always be directly linked to a well-defined chiral anomaly. This invites a reconsideration of how to interpret experimental data in the context of the chiral anomaly's contribution to transport phenomena in WSMs. Specifically, the findings highlight the need for cautious differentiation between contributions from the intrinsic properties of the electronic band structure and extrinsic factors like current distribution anomalies.

Regarding future developments, these findings imply that further investigations into the transport properties of Weyl and Dirac semimetals under varying Fermi energy levels could provide valuable insights into the interplay between topological electronic properties and classical transport responses.

Technical Findings

The paper documents a series of meticulous quantum oscillation measurements. These include Shubnikov–de Haas (SdH) and de Haas–van Alphen (dHvA) oscillations, which, when analyzed, provide detailed insight into the Fermi surface geometry and electronic band structure near the Weyl points. Notably, the Fermi energy was deduced to be slightly above the charge-neutral point, indicative of minor electron doping in the material.

Moreover, the researchers identify a set of angular-dependent quantum oscillation frequencies compatible with the theoretically predicted Fermi surface derived from the band structure. These findings are crucial for validating the theoretical models of the band structure of WSM like TaP.

Conclusion

The discussion in this paper sheds light on the complexity of associating negative MR solely with the chiral anomaly, especially in systems like TaP. At an applied level, understanding these nuances in WSMs could significantly impact the development of future electronic and optoelectronic devices leveraging topological properties. The research opens pathways for deeper examinations into connections between band topology and transport phenomena, potentially informing new physics in topological condensed matter systems.